Factors Of Evolution - Selection
( Originally Published 1909 )
ALTHO concerning the truth of descent there is now no doubt in the minds of biologists, there are many and various views as to the causes or factors which have brought about the origin of species of plants and animals. Most prominent of the theories concerning the factors of evolution is that of Natural Selection. This theory was conceived independently by Darwin and Wallace, and strange to say, the idea came to both from the perusal of the same book, Malthus 'say on Population.'
On reading this book in 1838 Darwin first conceived the idea of natural selection. But he writes, "I was so anxious to avoid prejudice that I determined not for some time to write even the briefest sketch of it. In June, 1842, I first allowed myself the satisfaction of writing a very brief abstract of my theory in pencil, in thirty-five pages, and this was enlarged during the summer of 1844 into one of 230 pages."
And Wallace writes : "In February, 1858, I was suffering from a rather severe attack of intermittent fever at Ternate, in the Moluccas, and one day while lying on my bed during the cold fit something led me to think of the 'positive checks' described by Malthus in his 'Essay on Population,' a work I had read several years before and which had made a deep and permanent impression on my mind. These checks . . . must, it occurred to me, act on animals as well as man . . . and while pondering vaguely on this fact, there suddenly flashed upon me the idea of the survival of the fittest. In the two hours that elapsed before my ague fit was over I had thought out almost the whole of the theory, and the same evening I sketched the draft of my paper and in the two succeeding evenings wrote it out in full and sent it by the next post to Mr. Darwin."
It thus appears, to use Professor Lacy's words, "that the announcement of the Darwin Wallace theory of natural selection was made in 1858 and in the following year was published the book, the famous `Origin of Species,' upon which Darwin had been working when he received Mr. Wallace's essay." The theory has come to bear Darwin's name and is frequently referred to as `Darwinism.'
From time to time there appear articles and pamphlets bearing many titles announcing the decline of Darwinism, and in many periodicals this is often taken to mean that scientific men are giving up the theory of descent or evolution. Not at all. The truth is simply that some scientific men do not accept Darwin's natural selection explanation of the causes which brought about the descent of species. But Darwin's whole theory of natural selection might be discarded without injuring the general theory of descent, for the facts outlined in preceding pages have no necessary relation to theories as to causes natural selection or other theories.
The confusion on this point is doubtless emphasized in the popular mind because Darwin not only set forth his natural selection theory in his book, the full title of which is 'The Origin of Species by Means of Natural Selection; or, The Preservation of Favored Races in the Struggle for Life,' but he also argued for the truth of evolution so well that it came to be generally accepted. However, the general idea of descent was clearly mapped out long before Darwin's time and he simply put the evidence into more convincing form. Hence evolution (the fact) is not Darwin's theory, but natural selection (a factor) deserves to bear his name.
In order to grasp the full significance of natural selection, it is first necessary to appreciate the astounding facts relating to the struggle for existence. On this point Darwin's classical statement deserves to be given. "The elder De Candolle and Lyell," he said, "have largely and philosophically shown that all organic beings are exposed to severe competition. Nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult than constantly to bear this conclusion in mind. Yet unless it be thoroly ingrained in the mind, the whole economy of nature, with every fact on distribution, rarity, abundance, extinction and variation, will be dimly seen or quite misunderstood. We behold the face of nature bright with gladness, we often see superabundance of food; we do not see or we forget that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings are destroyed by birds and beasts of prey; we do not always bear in mind that altho food may be now superabundant, it is not so at all seasons of each recurring year.
"I should premise that I use the term 'struggle for existence' in a large and metaphorical sense, including dependence of one being on another and including not only the life of the individual, but success in leaving progeny. Two canine animals, in a time of dearth, may be truly said to struggle with each other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought, tho more properly it should be said to be dependent on the moisture. A plant which ananually produces a thousand seeds, of which only one of an average comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The mistletoe is dependent on the apple and a few other trees, but can only in a far fetched sense be said to struggle with these trees, for, if too many of these parasites grow on the same tree, it languishes and dies. But several seedling mistletoes, growing close together on the same branch, may more truly be said to struggle with each other. As the mistletoe is disseminated by birds, its existence depends on them ; and it may metaphorically be said to struggle with other fruit-bearing plants in tempting the birds to devour and thus disseminate its seeds. In these several senses, which pass into each other, I use for convenience' sake the general term of Struggle for Existence.
"A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being which during its natural lifetime produces several eggs or seeds must suffer destruction during some period of its life and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life. Altho some species may be now in-creasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them.
"There is no exception to the rule that every organic being naturally increases at so high a rate that, if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty five years, and at this rate in less than a thousand years there would literally not be standing room for his progeny. Linnaeus has calculated that if an annual plant produced only two seeds and there is no plant so unproductive as this and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when thirty years old and goes on breeding till ninety years old, bringing forth six young in the interval and surviving till one hundred years old; if this be so, after a period of from 740 to 750 years there would be nearly nineteen million elephants alive, descended from the first pair.
"But we have better evidence on this subject than mere theoretical calculations, namely, the numerous recorded cases of the astonishingly rapid increase of various animals in a state of nature, when circumstances have been favorable to them during two or three following seasons. Still more striking is the evidence from our domestic animals of many kinds which have run wild in several parts of the world. If the statements of the rate of increase of slow-breeding cattle and horses in South America, and latterly in Australia, had not been well authenticated, they would have been incredible. So it is with plants; cases could be given of introduced plants which have become common throughout whole islands in a period of less than ten years.
"There are plants which now range in India from Cape Comorin to the Himalaya which have been imported from America since its discovery. In such cases, and endless others could be given, no one supposes that the fertility of the animals or plants has been suddenly and temporarily increased in any sensible degree. The obvious explanation is that the conditions of life have been highly favorable, and that there has consequently been less destruction of the old and young, and that nearly all the young have been enabled to breed. Their geometrical ratio of increase, the result of which never fails to be surprising, simply explains their extraordinarily rapid increase and wide diffusion in their new homes.
"In a state of nature almost every full-grown plant annually produces seed, and among animals there are very few which do not annually pair. Hence we may confidently assert that all plants and animals are tending to increase at a geometrical ratio that all would rapidly stock every station in which they could anyhow exist and that this geometrical tendency to increase must be checked by destruction at some period of life. Our familiarity with the larger domestic animals tends, I think, to mislead us ; we see no great destruction falling on them, but we do not keep in mind that thousands are annually salughtered for food, and that in a state of nature an equal number would have somehow to be disposed of,
"The only difference between organisms which annually produce eggs or seeds by the thousand and those which produce extremely few is that the slow-breeders would. require a few more years to people, under favorable conditions, a whole district, let it be ever so large. The condor lays a couple of eggs and the ostrich a score, and yet in the same country the condor may be the more numerous of the two; the Fulmar petrel lays but one egg, yet it is . believed to be the most numerous bird in the world. One fly deposits hundreds of eggs and another, like the hippobosca, a single one; but this difference does not determine how many individuals of the two species can be supported in a district. A large number of eggs is of some importance to those species which depend on a fluctuating amount of food, for it allows them rapidly to increase in number.
"But the real importance of a large number of eggs or seeds is to make up for much destruction at some period of life; and this period in the great majority of cases is an early one. If an animal can in any way protect its own eggs or young, a small number may be produced, and. yet the average stock be fully kept up; but if many eggs. or young are destroyed, many must be produced or the species will become extinct. It would suffice to keep up the full number of a tree which lived on an average for a thousand years if a single seed were produced once in a thousand years, supposing that this seed were never destroyed and could be insured to germinate in a fitting place. So that, in all cases, the average number of any animal or plant depends only indirectly on the number of its eggs or seeds.
"In looking at Nature, it is most necessary to keep the foregoing considerations always in mind never to forget that every single organic being may be said to be striving to the utmost to increase in numbers ; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old during each generation or at recurrent intervals. Lighten any check, mitigate the destruction ever so little, and the number of the species will almost instantaneously increase to any amount."
Many marvelous facts concerning the rapid increase of plants and animals have been collected since Darwin's time. Jordan and Kellogg give many instances and among them the following: "It is said that the conger eel lays 15,000,000 eggs yearly. If each hatched and the conger grew to maturity, in a few years there would be no room for any other kind of fish in the sea. The codfish has been known to produce 9,100,000 eggs each year. If each egg were to develop, in ten years the sea would be solidly full of codfish. The female quinnat salmon of the Columbia ascends the river at the age of about four years and lays 4,000 eggs, after which she dies. Half these eggs develop into males. If each female egg came to maturity, we should have at the end of fifty years 8,000,000,000,000,000,000,000,000,000,000,000,000,000,000 female salmon and as many males as the offspring of a single pair. It takes about one hundred of these salmon to weigh a ton. Could all these fishes develop, in a very short time there would be no room for them in all the rivers of the North nor in all the waters of the sea."
The records of the spread of the English sparrow in America and of rabbits in Australia are splendid examples of the possible rate of multiplication. Professor Sidney Dickinson gives the following notes on the rabbit of Australia ; "The fecundity of the rabbit is amazing, and his invasion of remote districts swift and mysterious. Careful estimates show that under favorable conditions a pair of Australian rabbits will produce six litters a year, averaging five individuals each. As the offspring themselves begin breeding at the age of six months, it is shown that, at this rate, the original pair might be responsible in five years for a progeny of over twenty millions.- That the original score that were brought to the country have propagated after some such ratio, no one can doubt who has seen the enormous hordes that now devastate the land in certain districts. In all but the remoter sections the rabbits are now fairly under control ; one rabbiter with a pack of dogs supervises stations where one hundred were employed ten years ago, and with ordinary vigilance the squatters have little to fear. Millions of the animals have been killed by fencing in the water holes and dams during a dry season, whereby they died of thirst and lay in enormous piles against the obstructions they had frantic-ally and vainly striven to climb, and poisoned grain and fruit have killed myriads more."
These examples must suffice. Any other animal or plant would illustrate the same principle, for each increases at a rate which would make it cover the earth in a comparatively short time. That this is not the case is due to this check known as the Struggle for Existence. This was well known before Darwin's time, but he was the first to see its great importance in the development of plant and animal species.
The nature of the checks to increase is often complex. One has but to watch a bit of natural land to see them working. Climatic conditions play an important part in limiting the increase, and a wet season will make some crops exceedingly light and many insects very rare. Over-crowding limits the supply of food and other needful things, and hence some plants are 'choked' and many animals starve. Serving as prey to other animals causes enormous destruction of both animals and plants, for thou-sands of insects and other animal enemies must have food. Thus Darwin in an observational experiment noted that out of 357 weed seedlings under observation 295 were destroyed by insects alone. Various bacterial and other fungous diseases destroy countless thousands of plants and animals in a single day, and it is well known that various diseases slue to animal and vegetable parasites are great checks to increase in most species of vertebrate animals.
In some cases the struggle is with the elements, in others special enemies constitute the most important check. The history of the cottony cushion scale in America, described by Miall, offers a good example of a special check of this sort. "About the year 1880 the orange groves of South California seemed to be infected with a kind of leprosy. White patches appeared on the trunks and branches, which at length ran together and covered the bark, the leaves turned yellow, and no fruit could be ripened. The plague spread with rapidity, and all the time-honored remedies were found to be ineffectual. It was soon made known that the symptoms described were due to the attack of a scale-insect, known in America as the fluted scale, or cottony-cushion scale.
"This formidable plague gradually increased its range, in spite of the vigorous use of poisonous washes. It was not till 1888 that an effectual remedy was found. The late C. V. Riley, Entomologist to the United States Department of Agriculture, had anxiously considered the ways of checking the fluted scale. He found out that it came from Australia, where it infested the bushes called wattles (Acacias) ; that it had been introduced into California about 1868, probably on Acacias; that in its native country it was not a serious plague, altho no remedies were employed. He concluded that it must be kept down in Australia by natural enemies of some kind which did sot exist in California and that the real policy was to discover and import these remedies, whether parasitic or predatory.
"In 1888 Riley sent a trained entomologist, Albert Koebele, to visit Australia, examine the gardens and report. He found that the fluted scale, tho widespread, was kept in check by several insect foes. The most promising of these for Californian purposes was considered to be an Australian lady-bird known to naturalists as 'Vedalia cardinalis.' This beetle, both as larva and adult, greedily devours the scale insect and its eggs, preys upon no other species and is very prolific. Koebele was diligent in procuring an abundant supply of Vedalia. The lady birds were soon distributed and found plenty of occupation. In a year and a half they had practically rid California of the pest."
Nor is this case of the cottony-cushion scale alone. Any text-book of economic entomology will cite hundreds of cases of similar kinds. At the present time the United States entomologist, Dr. L. O. Howard, is searching for a special enemy supposed to keep in check the gipsy moth, now so destructive in Massachusetts.
Such are some of the influences known to affect plants and animals in the struggle for existence. It is clearly to be seen that the whole question of the relation of the individual to its environment that is, the problems of plant and animal ecology are involved in the struggle for existence and in the checks to the tendency to overwhelming increase. Science has only begun to understand these problems of ecology. However, the principle of the struggle for existence is not affected because its causes are not known. This much is certain : organisms tend to increase rapidly; an untiring struggle for existence exists; the vast majority of individuals perish as a result of this struggle with the environment the result being as Darwin believed that those organisms best fitted to their environment survive--i.e., the struggle for existence leads naturally to the survival of the fittest. This is natural selection. Nature meaning the sum total of the environmental factors which affect an organism selects those individuals best fitted to the environment. How this works in detail is best explained after some consideration of the variation of organ-isms.
The foundation of the Darwinian theory is the variability of species, and it is quite useless to attempt even to understand that theory, much less to appreciate the completeness of the proof of it, unless a clear conception of the nature and extent of this variability is obtained. Variation in the state of nature has been discussed by Alfred Russell Wallace in 'Darwinism.' Mr. Wallace shows in some detail that individual variability is a general character of all common and widespread species of animals or plants ; and, further, that this variability extends, so far as is known, to every part and organ, whether external or internal, as well as to every mental faculty, and that each part or organ varies to a considerable extent independently of other parts. Again, he shows by abundant evidence that the variation that occurs is very large in amount usually reaching to or 20 and sometimes even 25 percent. of the average size of the varying part.
But a few examples can be given to illustrate this feature of life. The variability of plants is notorious, being proved not only by the endless variations which occur whenever a species is largely grown by horticulturists, but also by the great difficulty that is felt by botanists in deter-mining the limits of species in many large genera. As an example may be taken the roses. In Mr. Baker's 'Revision of the British Roses' he includes under the single species Rosa canina the common dog rose no less than twenty eight named varieties distinguished by more or less constant characters and often confined to special localities, and to these are referred about seventy of the species of British and continental botanists.
Concerning individual variation, the distinguished botanist, Alp. de Candolle, made a special study of the oaks of the whole world and has stated some remarkable facts as to their variability. He declares that on the same branch of oak he has noted the following variations : In the length of the petiole, as one to three ; in the form of the leaf, being either elliptical or obovoid ; in the margin being entire, or notched, or even pinnatifid; in the extremity being acute or blunt; in the base being sharp, blunt or cordate; in the surface being pubescent or smooth; the perianth varies in depth and lobing ; the stamens vary in number, independently ; the anthers are mucronate or blunt; the fruit stalks vary greatly in length, often as one to three; the number of fruits varies; the form of the base of the cup varies; the scales of the cup vary in form; the proportions of the acorns vary ; the times of the acorns ripening and falling vary. Besides this, many species exhibit well-marked varieties which have been described and named.
Among birds copious evidence of variation is found. The most systematic observations on the individual variation of birds have been made by J. A. Allen in his remarkable memoir, 'On the Mammals and Winter Birds of East Florida.' He says : "The facts of the case show that a variation of from 15 to 20 per cent. in general size, and an equal degree of variation in the relative size of different parts, may be ordinarily expected among specimens of the same species and sex, taken at the same locality, while in some cases the variation is even greater than this." He then goes on to show that each part varies to a consider-able extent independently of the other parts, so that when the size varies, the proportions of all the parts vary, often to a much greater amount. The wing and tail, for ex-ample, besides varying in length, vary in the proportionate length of each feather, and this causes their outline to vary considerably in shape. The bill also varies in length, width, depth and curvature.
Even tho one has not been impressed with the great variety of individuals in the state of nature, variation. in domesticated plants and animals and the efficacy of artificial selection in producing varieties of the different species of plants and animals cannot have escaped notice. The following paragraphs are taken from Mr. Wallace's account of variation in domesticated plants and animals:
"Every one knows that in each litter of kittens or of puppies no two are alike. Even in the case in which several are exactly alike in colors, other differences are always perceptible to those who observe them closely. They will differ in size, in the proportions of their bodies and limbs, in the length or texture of their hairy covering and notably in their disposition. They each possess, too, an individual countenance, almost as varied when closely studied as that of a human being. The same thing occurs in the vegetable kingdom. All plants raised from seed differ more or less from each other. In every bed of flowers or of vegetables we shall find, if we look closely, that there are countless small differences in the size, in the mode of growth, in the shape. or color of the leaves, in the form, color or markings of the flowers, or in the size, form, color or flavor of the fruit. These differences are usually small, but are yet easily seen, and in their extremes are very considerable; and they have this important quality, that they have a tendency to be reproduced, and thus by careful breeding any particular variation or group of variations can be increased to an enormous extent appargntly to any extent not incompatible with the life, growth and reproduction of the plant or animal.
"The way this is done is by artificial selection, and it is very important to understand this process and its results. Suppose we have a plant with a small edible seed and we want to increase the size of that seed. We grow as large a quantity of it as possible, and when the crop is ripe we carefully choose a few of the very largest seeds or we may by means of a sieve sort out a quantity of the largest seeds. Next year we sow only these large seeds, taking care to give them suitable soil and manure, and the result is found to be that the average size of the seeds is larger than in the first crop and that the largest seeds are now somewhat larger and more numerous. Again sowing these, we obtain a further slight increase of size, and in a very few years we obtain a greatly improved race, which will always produce larger seeds than the unimproved race, even if cultivated without any special care. In this way all our fine sorts of vegetables, fruits and flowers have been obtained, all our choice. breeds of cattle or of poultry, our wonderful race-horses and our endless varieties of dogs. It is a very common but mistaken idea that this improvement is due to crossing and feeding in the case of animals, and to improved cultivation in the case of plants. Crossing is occasionally used in order to obtain a combination of qualities found in two distinct breeds, and also because it is found to increase the constitutional vigor; but every breed possessing any exceptional quality is the . result of the selection of variations occurring year after year and ac-cumulated in the manner just described. Purity of breed, with repeated selection of the best varieties of that breed, is the foundation of all improvement in our domestic animals and cultivated plants.
"The experience of breeders and cultivators proves that variation is the rule instead of the exception, and that it occurs, more or less, in almost every direction. This is shown by the fact that different species of plants and animals have required different kinds of modifications to adapt them to our use, and we have never failed to meet with variation in that particular direction, so as to enable us to accumulate it and so to produce ultimately a large amount of change in the required direction. Our gardens furnish us with numberless examples of this property of plants. In the cabbage and lettuce we have found variation in the size and mode of growth of the leaf, enabling us to produce by selection the almost innumerable varieties, some with solid heads of foliage quite unlike any plant in a state of nature, others with curiously wrinkled leaves like the savoy, others of a deep purple color used for pickling. From the very same species as the cabbage have arisen the broccoli and cauliflower, in which the leaves have undergone little alteration, while the branching heads of flowers grow into a compact mass forming one of our most delicate vegetables.
"The most remarkable varieties are afforded by the apple, and some account of these will be given as illustrating the effects of slight variations accumulated by selection. All our apples are known to have descended from the common crab of our hedges, and from this at least a thou-sand distinct varieties have been produced. These differ greatly in the size and form of the fruit, in its color, and in the texture of the skin. They further differ in the time of ripening, in their flavor, and in their keeping properties; but apple-trees also differ in many other ways.
"Coming now to our domesticated animals, we find still more extraordinary cases; and it appears as if any special quality or modification in an animal can be obtained if we only breed it in sufficient quantity, watch carefully, for the required variations, and carry on selection with patience and skill for a sufficiently long period. Thus; in sheep we have enormously increased the wool, and have obtained the power of rapidly forming flesh and fat; in cows we have increased the production of milk; in horses we have obtained strength, endurance, or speed, and have greatly modified size, form, and color ; in poultry we have secured various colors of plumage, increase of size, and almost perpetual egg-laying; and in dogs and pigeons marvelous changes have been effected."
The facts of individual variation from the viewpoint of a prominent American botanist will supplement the statements of Darwin and Wallace. Professor L. H. Bailey based his lectures on 'Plant Breeding' on the universal difference in nature. He says : "No two living things are exact counterparts, for no two are born into exactly the same conditions and experiences. Every living object has individuality; that is, there is something about it which enables the acute observer to distinguish it from all other objects, even of the same class or species. Every plant in a row of lettuce is different from; every other plant, and the gardener, when transplanting them, selects out, almost unconsciously, some plants which please him and others which do not. If one were to make the effort, he would find that it is possible to distinguish differences between every two spears of grass in a meadow or every two heads of wheat in a grainfield.
"All this is equivalent to saying that plants are infinitely variable. The ultimate causes of all this variation are beyond the purpose of the present discussion, but it must be evident to the reflective mind that these differences are the means of adapting the innumerable individuals to every little difference or advantage in the environment in which they live.
"If no two plants are anywhere alike, then it is mot strange if now and then some departure, more marked than common, is named and becomes a garden variety. We have been taught to feel that plants are essentially stable and inelastic, and that any departure from the type is an exception, and calls for immediate explanation. The fact is, however, that plants are essentially unstable and plastic, and that variation between the individuals must everywhere be expected. This erroneous notion of the stability of organisms comes of our habit of studying what we call species. We set for ourselves; a type of plant or animal and group about it all those individuals which are more like this type than they are like any other, and this group we name a species. Nowadays, the species is regarded as nothing more than convenient and arbitrary expression for classifying our knowledge of the forms of life, but the older naturalists conceived that the species is the real entity or unit in nature, and we have not yet wholly outgrown the habit of mind which was born of that fallacy.
"Nature knows nothing about species; she is concerned with the individual, the ultimate unit. This individual she molds and fits into the chinks of environment, and each individual tends to become the more unlike its birth mates the more the environments of the various individuals are unlike. I would impress upon you, therefore, the importance of the individual plant, rather than the importance of the species; for thereby we put ourselves as nearly as possible in a sympathetic attitude with Nature, and, resting upon the ultimate object of her concern, we are able to understand what may be conceived to be her motive in working out the problems of life."
It should be noted that Darwin did not give much attention to the causes of variation. He simply gathered together the data to show that organisms are highly variable. Given the facts of the high variability of individuals of all plants and animals, and the intense struggle for existence, how will this struggle act in regard to variation? Can the principle of selection, which is seen to be so potent in the hands of man in artificial selection, apply to organisms in a state of nature? These are essentially the questions which Darwin asked, and his own answer, now become a classic in science, is still the most satisfactory.
"I think we shall see," he says, "that it (the principle of selection) can act most efficiently. Let the endless' number of slight variations and individual differences occurring in our domestic productions, and in a lesser degree in those under nature, be borne in mind; as well as the strength of the hereditary tendency. Under domestication, it may be truly said that the whole organization becomes in some degree plastic. But the variability, which we almost universally meet with in our domestic productions, is not directly produced, as Hooker and Asa Gray have well remarked by man; he can neither originate varieties nor prevent their occurrence; he can only preserve and accumulate such as do occur. Unintentionally he exposes organic beings to new and changing conditions of life, and variability ensues; but similar changes of conditions might and do occur under nature. Let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life; and consequently what infinitely varied diversities of structure might be of use to each being under changing conditions of life.
"Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life should occur in the course of many successive generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favorable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest. Variations neither useful nor injurious' would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions.
"We have good reason to believe that changes in the conditions of life give a tendency to increased variability; and this would manifestly be favorable to natural selection, by affording a better chance of the occurrence of profitable variations. Unless such occur, natural selection can do nothing. Under the term of 'variations' it must never be forgotten that mere individual differences are included. As man can produce a great result with his domestic animals and plants by adding up in any given direction individual differences, so could natural selection, but far more easily, from having incomparably longer time for action.
"Nor do I believe that any great physical change, as of climate, or any unusual degree of isolation to check migration, is necessary in order that new and unoccupied places should be left, for natural selection to fill up by improving some of the varying inhabitants. For as all the inhabitants of each country are struggling together with nicely balanced forces, extremely slight modifications in the structure or habits of one species would often give it an advantage over others; and still further modifications of the same kind would often still further increase the advantage, as long as the species continued under the same conditions of life and profited by similar means of subsistence and defense.
"As man can produce, and certainly has produced, a great result by his methodical and unconscious means of selection, what may not natural selection effect? Man can act only on external and visible characters. Nature, if I may be allowed to personify the natural 'preservation or survival of the fittest, cares nothing for appearances, except in so far as they are useful to any being. She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his own good: Nature only for that of the being which she tends. Every selected character is fully exercised by her, as is implied by the fact of their selection. Man keeps the natives of many climates in the sane country; he seldom exercises each selected character in some peculiar and fitting manner; he feeds a long and a short beaked pigeon on the same food; he does not exercise a long-backed or long-legged quadruped in any peculiar manner; he exposes sheep with long and short wool to the same climate. He does not allow the most vigorous males to struggle for the females. He does not rigidly destroy all inferior animals, but protects during each varying season, as far as lies in his power, all his productions. He often begins his selection by some half-monstrous form; or at least by some modification, prominent enough to catch the eye or to be plainly useful to him. Under nature, the slightest differences of structure or constitution may well turn the nicely balanced scale in the struggle for life, and so be preserved. How fleeting are the wishes and efforts of man ! how short his time ! and consequently how poor will be his results, compared with those accumulated by Nature during whole geological periods ! Can we wonder, then, that Nature's productions should be far 'truer' in character than man's productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship?
"It may metaphorically be said that natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress until the hand of time has marked the lapse of ages, and then so imperfect is our view into long-past geological ages that we see only that the forms of life are now different' from what they formerly were.
"In order that any great amount of modification should be effected in a species, a variety when once formed must again, perhaps after a long interval of time, vary or pre-sent individual differences of the same favorable nature as before; and these must be again preserved, and so onward step by step. Seeing that individual differences of the same kind perpetually recur, this can hardly be considered as an unwarrantable assumption. But whether it is true, we can judge only by seeing how far the hypothesis accords with and explains the general phenomena of nature. On the other hand, the ordinary belief that the amount of possible variation is a strictly limited quantity is likewise a simple assumption.
"Altho natural selection can act only through and for the good of each being, yet characters and structures, which we are apt to consider as of very trifling importance, may thus be acted on. When we see leaf-eating insects green, and bark-feeders mottled-gray, the alpine ptarmigan white in winter, the red grouse the color of heather, we must believe that these tints are of service to these birds and insects in preserving them from danger. Grouse, if not destroyed at some period of their lives; would increase in countless numbers ; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their prey-so much so that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction. Hence natural selection might be effective in giving the proper color to each kind of grouse, and in keeping that color, when once acquired, true and constant. Nor ought we to think that the occasional destruction of an animal of any particular color would produce little effect; we should re-member how essential it is in a flock of white sheep to destroy a lamb with the faintest trace of black.
"In looking at many small points of difference between species, which, as far as our ignorance permits us to judge, seem quite unimportant, we must not forget that climate, food, etc., have no doubt produced some direct effect. It is also necessary to bear in mind that owing to the law of correlation, when one part varies, and the variations are accumulated through natural selection, other modifications, often of the most unexpected nature, will ensue.
"As we see that those variations which, under domestication, appear at any particular period of life, tend to reappear in the offspring at the same period for instance, in the shape, size and flavor of the seeds of the many varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silk-worm; in the eggs of poultry, and in the color of the down of their chickens; in the horns of our sheep and cattle when nearly adult so in a state of nature natural selection will be enabled to act on and modify organic beings at any age, by the accumulation of variations profit-able at that age, and by their inheritance at a corresponding age.
"If it profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty in this being effected through natural selection than in the cotton planter increasing and improving by selection the down in the pods on his cotton-trees. Natural selection may modify and adapt the larva of an insect to a score of contingencies wholly different from those which concern the mature insect; and these modifications may effect, through correlation, the structure of the adult. So, conversely, modifications in the adult may affect the structure of the larva; but in all cases natural selection will ensure that they shall not be injurious; for if they were so, the species would become extinct.
"Natural selection will modify the structure of the young in relation to the parent, and of the parent in relation to the young. In social animals it will adapt the structure of each individual for the benefit of the whole community, if the community profits by the selected change. What natural selection cannot do is to modify the structure of one species, without giving it any advantage, for the good of another species ; and tho statements to this effect may be found in works of natural history, I cannot find one case which will bear investigation. A structure used only once in an animal's life, if of high importance to it, might be modified to any extent by natural selection; for instance, the great jaws possessed by certain insects, used exclusively for opening the cocoon; or the hard tip to the beak of unhatched birds, used for breaking the egg.
"It may be well fiere to remark that with all beings there must be much fortuitous destruction, which can have little or no influence on the course of natural selection. For instance, a vast number of eggs or seeds are annually devoured, and these could be modified through natural selection only if they varied in some manner which protected them from their enemies. Yet many of these eggs or seeds would, perhaps, if not destroyed, have yielded individuals better adapted to their conditions of life than any of those which happened to survive. So again, a vast number of mature animals and plants, whether or not they be the best adapted to their conditions, must be annually destroyed by accidental causes, which would not be in the least degree mitigated by certain changes of structure or constitution which would in other ways be beneficial to the species.
'But let the destruction of the adults be ever so heavy, if the number which can exist in any district he not wholly kept down by such causes or again, let the destruction of eggs or seeds be so great that only a hundredth or a thousandth part are developed yet of those which do survive the best adapted individuals, supposing that there is any variability in a favorable direction, will tend to propagate their kind in larger numbers than the less well adapted. If the numbers be wholly kept down by the causes just indicated, as will often have been the case, natural selection will be powerless in certain beneficial directions; but this is no valid objection to its efficiency at other times and in other ways, for we are far from having any reason to suppose that many species ever undergo modification and improvement at the same time in the same area."
In concluding this sketch of Darwin's Natural Selection theory it should be noted that Darwin believed natural selection to be the most important, but not the sole, factor in producing new forms of organisms. His later followers, especially of the Weismann school, have held natural selection to b the all-sufficient factor of evolution. It will be necessary to return to this in discussing modern criticisms of Darwinism. Summarizing the essential points in the natural selection theory, Jordan and Kellogg say that: "Of all the various factors of organic evolution, the one which has been most relied on as the great determining agent is that called Natural Selection, the survival of the individuals best fitted for the conditions of life, with the inheritance of those species-forming adaptations in which fitness lies. The primal initiative is not in natural selection, but in variation germinal and individual. This may be slight variation (fluctuation) or large deviation (saltation), but in any case, all difference in species or race must first be individual. The impulse to change, once arisen, is continued through heredity. From natural selection arises the choice among different lines of descent, the adaptive tending to exclude the non-adaptive, while traits which are neither helpful nor hurtful, but simply indifferent, may be borne along by the cur-rent of adaptive characters. Finally, separation or isolation tends to preserve a special line of heredity from being merged in the mass which constitutes the parent stock or species. Without individual variation no change could take place; all organisms would be identical in structure."
Altho the Darwinian theory has been subject to no popularly recognised attack during the last quarter of a century, nevertheless there has been accumulating a mass of criticism of it. Only a sketch of some of the leading criticisms can be presented. These will be taken in large measure from Kellogg's 'Darwinism Today.'
"Among the critics of the selection theories," he says, "we must note two groups, differing in the character of their criticism more in degree than in kind, perhaps, but still importantly differing. One group denies in to any effectiveness or capacity for species-forming on the part of natural selection, while the other group, a larger one, sees in natural selection an effective factor in directing or controlling the general course of descent, holding it to adaptive lines, but denies it outright any such 'Allmacht' of species control as the more eager selectionists, the so called neo Darwinians or Weismannians, credit it with. This larger group of critics sees in natural selection an evolutionary factor capable of. initiating nothing, dependent wholly for any effectiveness on some primary factor or factors controlling the origin and direction of variation, but wholly capable of extinguishing all unadapted, unfit lines of development, and in this way of exercising decisive final control over the general course of descent i.e., organic evolution.
"The general impression left on one after a consider-able course of anti-Darwinian reading is that there is a very real and effective amount of destructive criticism for Darwinians to meet; and at the same time a curious paucity of satisfactory or at all convincing substitutionary theory offered by the anti-Darwinians to replace that which they are attempting to dethrone. The situation illustrates admirably the varying worth of a few facts. A few stubborn facts of the wrong complexion are fatal things for a theory; they are immensely effective offensive weapons. But these same few facts make a pitiable showing when they are called on to support a theory of their own.
"It was exactly the greatest part of Darwin's greatness, it seems to me, that he launched his theory only after making the most remarkable collection of facts yet gathered together in biological science by any one man. Testing his theory by applying to it successively fact after fact, group after group, and category after category of facts, he convinced himself of the theory's consonance with all this vast array of observed biological actuality.
"Compare the grounding of any of the now offered replacing theories with the preparation and founding of Darwinism. In 1864 von Kölliker, a great biologist, convinced of the incapacity of natural selection to do the work assigned it by its founders and friends, suggested a theory of the origin of species by considerable leaps; in 1899 Korschinsky, on the basis of some few personal observations and the compiling of some others, definitely formulated a theory of species-forming by sudden considerable variations, namely, mutations ; in 1901 and 1903 appeared the two volumes of De Vries's 'Die Mutationstheorie,' in which are revealed the results of long years of careful personal observation, in truly Darwinian manner, directed toward the testing and better grounding of this mutations-theory of species-origin. The results are: out of many plant species studies a few show at certain times in the course of numerous generations a behavior in accordance with the demands of a theory of species-forming by sudden definitive modification; that is, species-forming by mutations.
"The mutations theory thus launcned is offered as a substitute for the natural selection theory obviously weakening under the fire of modern scientific criticism. But however effective De Vries's facts are in proving the possibility of the occurrence of other variations than those fortuitous ones occurring in continuous series from mean to opposite extremes which Darwin recognised as the basis of species-forming, and however effective they are in proving the actual production of three or six or ten species by mutation, and however effective in both these capacities they are as weapons of attack on the dominance of the Darwinian theory of species-making, how really inadequate are they to serve as the basis of a great all answering theory explaining in a causo mechanical way the facts of descent, or even the primary facts of general species-forming.
"The natural selection theory, as an all sufficient explanation of adaptation and species forming, has always had a weakness at its base; it depends absolutely, of ,course, on the preexistence of variations, but it itself has no influence whatever on the origin or control of these variations to give birth to other individuals. Now one of the chief problems in biology is exactly that of the origin, the causes, and the primary control of these con-genital variations.
"Three principal explanations, no one of them experimentally proved or even fairly tested as yet, have been given of this actually occurring congenital variation, viz., (1) that there exists in the germ plasm an inherent tendency or capacity to vary so that there is inevitable variation in all individuals produced from germ-plasm, this variation being wholly fortuitous and fluctuating according to some (the belief of Darwin and his followers), or, according to others, this variation following certain fixed or determinate lines (determinate variation, orthogenetic variation, etc.) ; (2) that amphimixis i.e„ biparental parentage is the principal cause of variation, it seeming logical to presume that individuals produced from germ-cells derived from the fusion of germ-plasm coming from two individuals more or less unlike would differ slightly from either of the parental individuals; and (3) that congenital variation is due to the influence of the ever-varying environment of the germ-cell producing individuals.
"The objections to any one of these theories may be very pertinent, as when one says regarding the first that calling a thing 'inherent' is not clearing up in any degree a phenomenon for which we are demanding a causo mechanical explanation ; or of the second; that it has been proved that individuals produced parthenogenetically that is, from an unmated mother vary, and in some cases vary even more than do other individuals of the same species produced by amphimixis; or of the third, that as far as our study of the actual processes and mechanism of the production of germ-cells and of embryos has gone.
We have found no apparent means whereby this influence of the ambient medium can be successfully impressed on the germ-plasm. But however pertinent the objections to the why of variation may be, they do not in any way invalidate the fact that variations do continuously and inevitably occur in all individuals, and that while many of these variations are recognisably such as have been impressed on the individual during its personal development as immediate results of varying temperature, amount or kind of food, degree of humidity, etc., to which it may be exposed in its young life, others seem wholly inexplicable on a basis of varying individual environment, and are certainly due to some antenatal influence acting on the germ-plasm from which the embryo is derived.
"Now, the natural selection theory, in its Darwinian and neo Darwinian form, presupposes fortuitously occur-ring congenital variations of practically infinite variety in all parts of all organisms. Actual observation shows that all parts of all organisms do vary, and that they vary congenitally ; that is, independently of any immediate influence during development exercised from with-out by environmental conditions, as well as in response, to these environmental influences, and finally, that in many cases this variation is fortuitous that is, that it occurs according to the laws of chance.
"The industrious statistical study of variations, including the tabulation of the variation condition in long series of individuals of the same species or race, and the mathematical formulation of this variation condition, have shown that in many specific cases, studied in numerous kinds of animal and plant forms, the character of the variation in any particular character may be truly rep-resented (with close approximation) by the mathematical expression and curve which would exactly define the condition in which the variation would exist if it actually followed the law of error. It is these continuous series of slight variations, these variously called fluctuating, individual, or Darwinian variations, occurring in all organisms at all times, and often following in their occurrence the laws of chance, on which Darwin's theory of species-forming by natural selection is based.
"But this same industrious statistical and quantitative study of variation which has proved that some variations do occur regularly, fluctuating around a mean or mode, has shown, as well, that in many cases the variations distinctly tend to heap up on one side or the other of the mean; that is, that they tend to occur along certain lines or toward certain directions rather than uniformly out in all directions. Also it is true and this has, of course, been long known that by no means all variations are so slight nor in such perfectly gradatory or continuous series as is true of the gradatory Darwinian variations. 'Sports' have been known to breeders of plants and animals ever since plant and animal breeding began. Bateson has filled a large book with records of 'discontinuous variations' in animals ; variations, that is, of large size and not occurring as members of continuous gradatory series. So that biologists are acquainted with many cases of variation that seem to be of a kind, or to exhibit a tendency to institute special directions of development, and thus not to be of the simple, noninitiating, inert character of the fortuitous, slight, fluctuating variations, among which natural selection is presumed to choose those that are to become the beginnings of new lines of modification and descent. Many biologists believe firmly that variations occur in many special cases, if not in most cases, only along certain special lines. Paleontologists believe, practically as a united body, that variation has followed fixed lines through the ages; that there has been no such unrestricted and utterly free play of variational vagary as the Darwinian natural selection theory presupposes.
"Now it is at least obvious that natural selection is absolutely limited in its work to the material furnished by variation, so that if variation occurs in any cases along certain determinate lines selection can do no more than make use of these lines. Indeed, if variation can occur persistently along determinate lines, natural selection's function in controlling evolution in such cases is limited to the police power of restricting or inhibiting further development along any one or more of these lines which are of a disadvantageous character; that is, a character which handicaps or destroys the efficiency of its members in the struggle for life. The question in many men's mouths to-day is, Why may not variation be the actual determinant factor in species-forming, in descent? It actually is, respond many biologists and paleontologists. Even Darwin believed such determinate variation to occur, as is indicated by repeated statements in the 'Origin of Species.'"
This problem of the existence or non-existence of de-terminate variations is one of the most important matters in connection with the whole great problem of descent; that is, of evolution. It is the basic problem of evolution, for it is the problem of beginnings. Selection, isolation, and the like factors are conditions of species-forming; variation is a prerequisite. True variation must have its causes, and these causes are to be determined before an actual causo mechanical explanation of evolution can ever be found. But the determination of the relation of variation to species-forming is certainly the first step now necessary in our search for the basic factors, the real first cause of species change.
Space will not permit analysis of the general lines of objections raised in the foregoing. These objections have led many biologists to the totally ante-Darwinian position that the struggle for existence and the corresponding selection of the fit are not factors of evolution. But perhaps the majority of biologists who recognise the objections cited are inclined to belief in natural selection as the great 'conserving factor of evolution' while allowing that it does not create new forms. To quote from T. H. Morgan, ". . . the theory of natural selection has nothing to do with the origin of species, but with the survival of already formed species. Not selection of the fittest individuals, but the survival of the sufficiently fit species."
In summarizing the present-day standing of the Darwinian theory, Kellogg says: "I think I speak fairly in saying that the believers and defenders of the natural selection theory today admit in large measure the validity of those criticisms which are directed at the incapacity of Darwinism, in its long familiar form, to account for the development of variations and modifications up to the advantageous or disadvantageous stage. They admit, also, the actual existence, and in abundant measure, of species differences which are of indifferent character that is, of no special utility and make the consequent; admission that such species differences cannot for the most part be explained by natural selection. And they also concede, or at least most of them, including Weis mann, do, the force of the criticism that the assumption of the occurrence of the right variations at the right time is a necessity for the development by selection of many, if not most, specializations of qualitative and of coadaptive character, which assumption in turn demands an explanation of causes anterior to selection.
"And finally, most selectionists concede that selection cannot make new species by relying on the extremes of series of fluctuating or Darwinian variations, because of the inevitable extinguishing or swamping of these extreme variations by interbreeding with the far more abundant average or modal individuals of the species."
Making such concessions, it is necessary to recognise that there are factors of evolution other than natural selection. and these require separate treatment.