Farm Animals - Origin Of The Hereditary Material
( Originally Published 1912 )
The phenomena discussed in the preceding pages give some conception of the fundamental nature of the forces to be dealt with. They suggest explanations of occurrences otherwise perplexing, but manifestly before we can undertake to formulate means to purify the hereditary material we must know something of its source. A consideration of the relation of that substance to the parent body is in order. The germ cells were traced from their rudimentary stages in the ovaries and testicles, but from whence did these organs derive this material of such extraordinary potency ? We have arrived at the end of our positive knowledge of hereditary processes. No examination or experiment has as yet revealed all the facts regarding the immediate source of thecontents of the sexual cells. Our embryologists explain development of the tissue layers and later of separate organs from the fertilized germ cell. But as to just what goes into the new reproductive organs there is no definite knowledge, though in the embryonic development of some lower forms there is evidence that an early segregation from the total parental germ plasm isolates a portion for reserve in the new reproductive organs while the main amount is dissipated in the building of the body. In view of the great desirability of understanding the origin of this vital substance, the best that scientists can do is to theorize. That theory which accords with the facts and which best accounts for the various manifestations of heredity is the one which will he of greatest service.
Darwin believed that the reproductive organs acted in somewhat the same manner as do the secreting organs of the body, the material they secrete consisting of minute particles that enter the blood circulation from all the cells of the body and are withdrawn, in the ovaries of the female or testicles of the male, and built up into the ova or spermatozoa. Weismann considers that the hereditary material is not drawn from the body but that rather a small proportion of the same material received from the parents is reserved intact in the reproductive organs and there remains until the animal reaches breeding age and then be-comes active and produces the germ cells.
These are the two main ideas on the subject. Actual examination or experiment to determine the facts seems impossible. We are therefore forced to base our practice on that explanation which seems most satisfactorily to explain the occurrences. If Darwin's suggestion be accepted we must chiefly emphasize the individuality of the parents rather than the ancestry, while the reverse is true if we think with Weismann.
In view of the fundamental importance of an intelligent idea of the source of the hereditary substance it is desirable and fair to more explicitly present the views of the two scientists referred to. Darwin's theory is known as "Pangenesis" and in his "The Variation of Plants and Animals under Domestication" he outlines his proposed explanation of the method of heredity in these words :
"It is universally admitted that the cells or units of the body increase by self-division or proliferation, retaining the same nature, and that they ultimately become converted into various tissues and substances of the body. But besides this means of increase I assume that the units throw off minute granules which are dispersed through-out the whole system ; that these, when supplied with proper nutriment, multiply by self-division and are ultimately developed into units like those from which they were originally derived. These granules may be called `gemmules'. They are collected from all parts of the system to constitute the sexual elements and their develop-ment in the next generation forms a new being; but they are likewise capable of transmission in a dormant state to future generations and may then be developed. Gem-mules are supposed to be thrown off by every unit, not only during the adult state, but during each stage of development of every organism; but not necessarily during the continued existence of the same unit. Lastly, I assume that the gemmules in their dormant state have a mutual affinity for each other, leading to their aggregation into buds, or into the sexual elements. Hence, it is not the reproductive organs, or buds, which generate a new organism, but the units of which each individual is composed. These assumptions constitute the provisional hypothesis which I have called `Pangenesis.' " He later states : "I am aware that my view is merely a provisional hypothesis or speculation; but, until a better one be advanced, it will serve to bring together a multitude of facts which are at present left disconnected by any efficient cause."
Weismann's hypothesis is in the main the exact opposite of that of Darwin. He designates the chromatin or hereditary material as "germ " His idea of "Continuity of Germ Plasm" regards the hereditary material as passing from generation to generation with the minimum of influence from, or association with the bodies of the parents. He regards the ovaries and testicles as depositories of hereditary material. In them is deposited at an early stage of embryonic life, a part of the germ plasm, there to be retained intact until its host arrives at the age for reproduction. Before reproduction is rendered possible this dormant material within the organs quickens into activity; it absorbs food material from the circulation of its host, increases its volume and completes the various processes already explained as essential to the production of ova or spermatozoa. While at first thought it may appear strange to think of this germ plasm as living unmodified in the body from which it derives its support for increase, yet it is no more strange than the fact that widely different classes of plants draw from a particular soil those elements they need, and by virtue of its inherent tendencies each constructs the material into a new plant strictly of the ancestral type and but very slightly modified, if at all, by the medium in which it has its root. Even so may the germ plasm in the reproductive organs increase in quantity without changing materially its quality.
In the manner of its behavior subsequent to fertilization, in diffusing throughout the embryo and dominating every cell, the germ plasm is quite comparable to the yeast plant. What corresponds to the yeast supply is within the reproductive organs and is there perpetuated much like a parasitic growth, and periodically sends off portions of itself to grow and diffuse through a whole new organism just as the small portion of yeast multiplies, acts upon the flour in each batch of dough and changes it to a quite different product, while the yeast supply is continued indefinitely by affording the favor-able conditions to the smallest amount of the original stock. The buttermaker carrying a good starter for a long period affords another analogy.
The many changes which animals undergo in the course of time would be accounted for by Weismann on the basis of selection from those departures or innovations occasioned by the necessity of reproduction by sexes, which process we have studied under the heads of maturation and fertilization. These vital considerations associated with the preparation and union of sexual cells as set forth in chapter five must not be confused with theory; they are fully demonstrated facts. It will be recalled that it was Weismann who saw the necessity for reducing division and predicted such a discovery before it was actually made.
The Darwinian theory would regard the germ cell as the epitome or concentration of the parent. While this might be conceivable for the production of most of the organs, a difficulty is encountered in the common case of persistence of lambs' tails in flocks in which sheep have been docked in early life for scores of generations. Dar-win was not unmindful of this difficulty and met it by supposing the transmission of dormant gemmules carried down from early ancestors not docked, in sufficient numbers to reproduce such parts, of which the absence in parents would otherwise preclude their presence in the offspring because of the impossibility of such gemmules being present. In this case and in many similar ones, the influence of such dormant gemmules preserved from re-mote parents seems to be the rule rather than the exception. Weismann would regard the continuation of such characters not used or not present in the parents as regular and to be expected. Indeed his idea of continuity readily explains the persistence of such apparently need-less parts as the vermiform appendage and the chestnuts on the legs of horses. In earlier forms these structures were doubtless functional. Under the Darwinian idea their disappearance and recurrence in offspring could only occur through dormant gemmules, as the exception ; under the Weismann theory such occurrences would be the rule rather than the exception, as we know they are in nature.
If, however, we incline strongly to the theory of continuity of germ plasm we are apparently cut off from all possibility of the reflection in the offspring of even extreme conditions affecting their parents. It is admitted, however, that in extreme cases the lack or abundance of a specific substance in the system of the parent body may either retard or facilitate the multiplication within the primary cell of some part of the germ plasm dependent upon such specific substance. Of course, in case of under-support of such hereditary elements they would not be entirely excluded from the germ cells, but the same conditions obtaining for several successive generations would have a positive influence toward weakening such tendencies just as the opposite kind of support might strengthen them. This is the only provision made by Weismann for direct influence of environment upon heredity.* All else is due to the selection of the parents, governed either by natural demands or the artificial considerations obtaining in domestic animals. The selection of parents for their valuable qualities constitutes continual opportunity for modifying the make-up of germ plasm of the succeeding generations.
Breeders who adhere to the idea contained in pangenesis would naturally judge of an animal's value as a breeder altogether from the charac ters he individually exhibits. His ancestry would be of interest only for chance of conveyance of dormant gemmules which would not be of more than very secondary importance. Although some breeders often permit what they deem a good line of ancestry to outweigh the individual characters of an animal, yet we must recognize the fact that all of our experienced and more successful breeders have been close students of pedigrees and their estimation of an animal's breeding powers has been based in large part on a knowledge of his ancestors. In so doing they have shown an appreciation of the chief principle of the Weismann idea, namely, that to mold our animals we must rely on changing the germ plasm by infusions by mating rather than by seeking to modify that substance by the influence of external conditions or being guided solely by external appearances. A conception of the nature of the germ plasm contained in an individual must be based upon a knowledge of the ancestors from whom that germ plasm was obtained no less than upon individual appearances. The practical significance of this principle is the subject of the next chapter.