Evolution and Adaptation

by Thomas Hunt Morgan

The study of bud variation, of the various forms of inheritance, and of reproduction and of the causes of variation, led him, Darwin says, to the belief that these subjects stand in some sort of relation to each other. He says: “I have been led, or rather forced, to form a view which to a certain extent connects these facts by a tangible method. Every one would wish to explain to himself, even in an imperfect manner, how it is possible for a character possessed by some remote ancestor suddenly to reappear in the offspring; how the effects of increased or decreased use of a limb can be transmitted to the child; how the male sexual element can act not solely on the ovules, but occasionally on the mother form; how a hybrid can be produced by the union of the cellular tissue of two plants independently of the organs of generation; how a limb can be reproduced on the exact line of amputation, with neither too much nor too little added; how the same organism may be produced by such widely different processes, as budding and true seminal generation; and, lastly, how of two allied forms, one passes in the course of its development through the most complex metamorphoses, and the other does not do so, though when mature both are alike in every detail of structure. 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.”

In presenting the hypothesis of pangenesis Darwin begins by enumerating the different kinds of sexual and asexual processes of reproduction, for which he hopes to offer a provisional explanation. Here we find mentioned various methods of budding and self-division, regeneration, parthenogenesis, sexual reproduction, and the inheritance of acquired characters. It is with the last only that we are here chiefly concerned; in fact, the need of an hypothesis of this sort to explain the other kinds of inheritance is by no means evident. There are, however, two other phenomena, besides that of the supposed inheritance of acquired characters, to which the hypothesis of pangenesis might appear to apply specially, namely, the effect of foreign pollen on the tissues of the mother plant, and the supposed influence of the union with the first male on the subsequent young (telegony). It is, however, far from being shown that any influence of this latter kind really occurs, despite the fact that it is generally believed in by breeders.

It is important to observe that Darwin proposes to explain on the hypothesis of pangenesis, not only the inheritance of characters acquired through use, but also the decrease of structures through disuse; and this applies, not only to the structure, but to function as well, as when the intelligence of the dog is explained through his association with man, and the tameness of the domestic rabbits through their long confinement. In the following quotation these points are referred to: “How can the use or disuse of a particular limb or of the brain affect a small aggregate of reproductive cells, seated in a distant part of the body, in such a manner that the being developed from these cells inherits the characters of either one or both parents? Even an imperfect answer to this question would be satisfactory.”

Coming now to the theory, we find that it consists of one chief assumption and several minor ones. “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 the 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 throughout 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 development 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.... Gemmules 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 new organisms, but the units of which each individual is composed. These assumptions constitute the provisional hypothesis which I have called Pangenesis.”

It will be noticed that the first assumption is that the cells throw off minute gemmules or granules. The second assumption is that these are collected in the reproductive organs, or in buds, or in regenerating parts; the third assumption is that the gemmules may lie dormant through several generations; the fourth, that the development of the reproductive cells is not so much the development of the cell itself, but of the gemmules that have collected in it. The fifth assumption is that the gemmules are thrown off at all stages of development; the sixth, that in their dormant state they have a mutual affinity for each other; the seventh, that there may be a sort of continual competition in the germ-cells between the original gemmules and the new ones, and, according to which win, the old or the new form develops. Thus we see on closer analysis that the pangenesis hypothesis is made up of a goodly number of different assumptions. At least half a dozen imaginary properties are ascribed to the imaginary gemmules, and these attributes are all essential to the working of the hypothesis.

Some of the more obvious objections to the hypothesis have been stated by Darwin himself. Such, for instance, as our ignorance at what stage in their history the body-cells are capable of throwing off gemmules, and whether they collect only at certain times in the reproductive organs, as the increased flow of blood to these organs at certain seasons might seem to indicate. Nor have we any evidence that they are carried by the blood at all. The experiment of Galton, of transfusing the blood of one animal into another, and finding that this produced no effect on the young that were born later, might be interpreted to mean that gemmules are not transported by the blood; but this kind of experiment is inconclusive, especially in the light of recent results on the effect of the blood of one animal on that of another.

A part of the evidence on which Darwin relied to support his theory has been shown to be incorrect by later work. Thus the assumption that more than a single pollen grain, or more than one spermatozoon, is necessary in some cases for fertilization, is certainly wrong. In most cases, in fact, the entrance of more than one spermatozoon into the egg is disastrous to the development. The cases referred to by Darwin can probably be explained by the difficulty that some of the pollen grains, or spermatozoa, may have in penetrating the egg, or to the immaturity or impotence of some of the male germ-cells, and not to the need of more than one to accomplish the true fertilization.

Darwin’s idea that the small number of gemmules in the unfertilized egg may account for the lack of power of such eggs to develop until they are fertilized, has been shown to be incorrect by recent results in experimental embryology. We now know that many different kinds of stimuli have the power to start the development of the egg. Moreover, we also know that if a single spermatozoon is supplied with a piece of egg-protoplasm without a nucleus, it suffices to cause this piece of protoplasm to develop.

In the case of regeneration, which Darwin also tries to explain on the pangenesis hypothesis, we find that there is no need at all for an hypothesis of this sort; and there are a number of facts in connection with regeneration that are not in harmony with the hypothesis. For instance, when a part is cut off, the same part is regenerated; but under these circumstances it cannot be imagined that the part removed supplies the gemmules for the new part. Darwin tries to meet this objection by the assumption that every part of the body contains gemmules from every other part. But it has been shown that if a limb of the newt is completely extirpated, a new limb does not regenerate; and there is no reason why it should not do so on Darwin’s assumption that germs of the limb exist throughout the body.

The best-authenticated cases of the influence of the male on the tissues of the female are those in plants, where one species, or variety, is fertilized by another. Thus, if the orange is fertilized by the pollen of the lemon, the fruit may have the color and flavor of the lemon. Now the fruit is a product of the tissues of the ovary of the female, and not a part of the seedling that develops in the fruit from the cross-fertilized egg-cell. Analogous cases are recorded for the bean, whose pods may have their color influenced by fertilizing the flower with pollen of another variety having pods of a different color. In these cases we do not know whether the color of the fruit is influenced directly by the foreign pollen, or whether the influence is through the embryo that develops from the egg-cell. The action may appear to be the same, however, in either case; but because it seems probable here that there is some sort of influence of one tissue on another, let us not too readily conclude that this is brought about through any such imaginary bodies as gemmules. It may be directly caused, for instance, by some chemical substance produced in the young hybrid plant. If this is the case, the result would not be different in kind from that of certain flowers whose color may be influenced by certain chemical substances in the soil.