Evolution and Adaptation

by Thomas Hunt Morgan

The growth of the antlers of stags is cited by Hurst in order to illustrate that what has been interpreted as a recapitulation may have a different interpretation. “Each stag develops a new pair of antlers in each successive year, and each pair of antlers is larger than the pair produced in the previous year. This yearly increase in the size of the antlers has been put forward as an example of an ontogenetic record of past evolution. I, however, deny that it is such a record.”

“The series of ancestors may have possessed larger antlers in each generation than in the generation before it. It is not an occasional accidental parallelism between the ontogeny and the phylogeny which I deny, but the causal relation between the two. Had the ancestors had larger antlers than the existing ones, there is no justification for the assumption that existing stags would acquire antlers of which each pair, in later years, would be smaller than those of the previous year.”

Hurst concludes: “There are many breeds of hornless sheep, but they do not bear large horns in early years and then shed them. If a rudiment ever appears in the embryo of such sheep, its growth is very early arrested.” The case of the appendix in man might have been cited here as a case in point. It is supposed to have been larger in the ancestors of man, but we do not find it appearing full size in the embryo and later becoming rudimentary. The preceding statements will show that, while Hurst’s view is similar in some respects to my own, yet it differs in one fundamental respect from it, and in this regard he approaches more nearly to the theory of Von Baer.

Hertwig has recently raised some new points of issue in regard to the recapitulation theory, and since he may appear to have penetrated farther than most other embryologists of the present time, it will be necessary to examine his view somewhat carefully. He speaks of the germ-cell (egg, or spermatozoön) as a species-cell, because it contains, in its finer organization, the essential features of the species to which it belongs. There are as many of these kinds of cells as there are different kinds of animals and plants. Since the bodies of the higher animals have developed from these species-cells, so the latter must have passed in their phylogeny through a corresponding development from a simple to a more and more complex cell-structure. “Our doctrine is, that the species-cell, even as the adult, many-celled representative of the species, has passed through a progressive, and, indeed, in general a corresponding development in the course of phylogeny. This view appears to stand in contradiction to the biogenetic law. According to the formula that Haeckel has maintained, the germ development is an epitome of the genealogy; or the ontogeny is a recapitulation of the phylogeny; or, more fully, the series of forms through which the individual organism passes during its development from the egg-cell to the finished condition is a short, compressed repetition of the longer series of forms which the forefathers of the same organism, or the stem-form of the species, has passed through, from the earliest appearance of organisms to the present time.” “Haeckel admits that the parallel may be obliterated, since much may be absent in the ontogeny that formerly existed in the phylogeny. If the ontogeny were complete, we could trace the whole ancestry.” Hertwig states further, that “The theory of biogenesis[^[12]] makes it necessary to change Haeckel’s expression of the biogenetic law, so that a contradiction contained in it may be removed. We must drop the expression ‘repetition of the form of extinct forefathers,’ and put in its place the repetition of forms which are necessary for organic development, and lead from the simple to the complex. This conception may be illustrated by the egg-cell.”

[12]. This term, by which Hertwig designates a particular view of his own, has been already preoccupied in a much wider sense by Huxley to mean that all life comes from preëxisting life. Hertwig means by the theory of biogenesis that as the egg develops there is a constant interchange between itself and its surroundings.

Since each organism begins its life as an egg we must not suppose that the primitive conditions of the time, when only single-celled amœbas existed on our planet, are repeated. The egg-cell of a living mammal is not, according to Hertwig’s hypothesis, an indifferent structure without much specialization like an amœba, but is an extraordinarily complex end-product of a long historical process, which the organized substance has passed through. If the egg of a mammal is different from that of a reptile, or of an amphibian, because in its organization it contains the basis of a mammal, just so much more must it be different from the hypothetical one-celled amœba, which has no other characteristics than those that go to make up an amœba. Expressed more generally, the developmental process in the many-celled organisms begins, not where it began in primitive times, but as the representation of the highest point which the organization has at present reached. The development commences with the egg, because it is the elemental and fundamental form in which organic life is represented in connection with the reproductive process, and also because it contains in itself the properties of the species in its primordia.

“The egg-cell of the present time, and its one-celled predecessor in the phylogenetic history, the amœba, are only comparable in so far as they fall under the common definition of the cell, but beyond this they are extraordinarily different from each other.”

“The phyletic series must be divided into two different kinds of processes:—First. The evolution of the species-cell, which is a steady advance from a simple to a complex organization. Second. The periodically repeated development of the many-celled individual out of the single cell, representative of the species (or the individual ontogeny), which in general follows the same rules as the preceding ontogeny, but is each time somewhat modified according to the amount to which the species-cell has itself been changed in the phylogeny. Similar restricting and explanatory additions to the biogenetic law, like those stated here for the one-celled stage, must be made in other directions. Undoubtedly there exists in a certain sense a parallel between the phylogenetic, and the ontogenetic, development.

“On the basis of the general developmental hypothesis on which we stand, all forms which in the chain of ancestors were end-products of the individual development are now passed through by their descendants as embryonic stages, and so in a certain degree are recapitulated. We also admit that the embryonic forms of higher animals have many points of comparison with the mature forms of related groups standing lower in the system.

“Nevertheless, a deeper insight into the conditions relating to these resemblances shows that there are very important differences that should not be overlooked. Three points need to be mentioned: 1. The cell-material which in the ancestral chain gives the basis for each ontogenetic process is each time a different material as far as concerns its finer organization and primordia. Indeed, the differences become greater the farther apart the links of the original chain become. This thought may be formulated in another way: The same ontogenetic stages that repeat themselves periodically in the course of the phylogeny always contain at bottom a somewhat different cell-material. From this the second rule follows as a consequence. 2. Between the mature end-form of an ancestor and the corresponding embryonic form of a widely remote descendant (let us say between the phylogenetic gastræa and the embryonic gastrula stage of a living mammal, according to the terminology of Haeckel) there exists an important difference, namely, that the latter is supplied with numerous primordia which are absent in the other, and which force it to proceed to the realization of its developmental process. The gastrula, therefore, as the bearer of important latent forces, is an entirely different thing from the gastræa, which has already reached the goal of its development. 3. In the third place, at each stage of the ontogeny outer and inner factors are at work, in fact even more intensely than in the fully formed organism. Each smallest change that acts anew in this way at the beginning of the ontogeny can start an impulse leading to more extensive changes in later stages. Thus the presence of yolk and its method of distribution in the egg alone suffice to bring about important changes in the cleavage, and in the formation of the germ-layers, the blastula, and gastrula stages,” etc. “Moreover, the embryo may adapt itself to special conditions of embryonic life, and produce organs of an ephemeral nature like the amnion, chorion, and placenta.”