CHAPTER III.

HISTORY OF THE THEORY OF HEREDITY—(Continued).

Some form of the evolution hypothesis a logical necessity— Darwin's pangenesis hypothesis—This is an evolution hypothesis, since all the characteristics of the adult are supposed to be latent in the germ-Miscellaneous objections to it-These objections do not show that it conflicts with fact-Difficulty in imagining detailed working is no reason for rejecting it -Galton's experimental disproof-There are many reasons for believing that the sexual elements have different functions -The evidence from parthenogenesis—Polar-cell hypothesis —The evidence from hybrids, from variation, and from structures confined to one sex-The pangenesis hypothesis recognizes no such difference in the functions of the reproductive elements-We must therefore distrust its absolute correctness -Summary of last two chapters.

Some Form of the Evolution Hypothesis a Logical

Necessity.

Most of the hypotheses which have been proposed, of late years, to account for the phenomena of heredity, are like the two we have quoted, epigenesis hypothesis, for they are attempts to show that the ovum is in reality, as well as in form, an unspecialized cell. Analysis shows, however, that they all rest ultimately upon the assumption that this is not true, but that the ovum really contains, in some form or other, actually or potentially, the future organism, with all its hereditary characteristics.

We know that eggs which are to all appearances essentially alike, may, when artificially removed from the ova

Erblichkeit ist das Gedächtniss der Plastidule, die Variabilität is die Fassungskraft der Plastidule). The one brings about the constancy and the other the diversity of organic forms. In the very simple and persistent forms of life the plastidules have, so to speak, learned nothing and forgotten nothing. In highly perfected and variable organisms the plastidules have both learned and forgotten much.”

This somewhat long quotation contains a thorough and exhaustive statement of the perigenesis hypothesis, and it is therefore interesting to notice that its only real claim to recognition as a true explanation of the phenomena of heredity is based upon or at least demands the acceptance of some form of the evolution hypothesis.

However great may be the importance of the analogy between the gradual evolution of the species by the specialization of the constituent individuals, and the development of the individual by the specialization of cells, and plastidules, we have already pointed out that it is in no sense an explanation of the latter, since the real cause of the evolution of the species, the selection of congenital variations, is absent.

The only part of Haeckel's hypothesis of perigenesis which has any claim to be considered an explanation of the reproductive power of animals, is the statement that heredity is memory, and variability the acquisition of new experiences. Stated by itself, without explanation, this may seem to those who are unfamiliar with the subject very much like nonsense, for the profound truth upon which it rests is not at all obvious at first sight.

Herbert Spencer has, in his masterly discussion of the nature and distinctive characteristic of life, given us, as the sum and substance of his analysis, the statement

that "life is the continuous adjustment between internal relations and external relations." This, like Haeckel's statement that heredity is memory, is not very clear without explanation, but its meaning may perhaps be brought out by an illustration.

If I kick a stone I produce in it certain changes, such as motion, heat, etc.; these changes being directly produced by the kick are simply manifestations of the energy transferred from my foot to the stone. If, instead of a stone, I kick a dog, I produce a similar set of changes, and something more. The experience of the dog and of his ancestors has taught him that such violent attacks are always associated with a disposition to commit still further violence, so, when the dog feels the blow he immediately performs actions which have as their object, escape from or avoidance of the danger which he has not yet experienced, but which he knows to be imminent. These actions are not the effect of the kick, for the energy expended may be hundreds of times greater. Their character is determined, not by any change in the dog, but by the character, the disposition, which he has inherited; and whether he retaliates by an attack on his own part, puts his tail between his legs and runs, or crouches at my feet, his actions are the effect, not of the kick, but of past experience as to the best means of escaping further injury. There is a relation, external to the dog, between the kick and a disposition to injure the dog, and there is within the dog a relation between the sensation of injury and the actions which experience has shown to be the proper ones for escaping further injury.

That which distinguishes the dog from the stone is the power to adjust these internal relations to the external relations, to conform his conduct to the laws of

the world around him. The dog, as a living thing, differs from all inorganic bodies, in his power to make this adjustment: so long as he retains this power he lives; his life is a "continuous adjustment between internal relations and external relations." It is plain that this power depends upon experience, but experience depends upon "memory." So we may state, with truth, that in a certain sense, life is memory; and as the power to reproduce its like is characteristic of all living things, we see that there is in Haeckel's statement a profound truth.

We know memory, however, only in connection with organization, and if it is true that heredity, the power of an organism to reproduce its like, is simply the memory, by the ovum, of the experience of its ancestors, we must believe that there exists in the ovum an organization of some kind to correspond to each of these past experiences.

We are therefore driven by the hypothesis of perigenesis back from the hypothesis of epigenesis to some form of the old evolution hypothesis, for we cannot conceive that complicated experiences should exist without complicated structure.

We are thus compelled to conclude that, while it undoubtedly expresses a great truth, Haeckel's hypothesis of perigenesis is not a satisfactory and final explanation of the phenomena of reproduction. A satisfactory theory of heredity must explain what it is, in the structure and organization of the ovum, which determines that each ovum should produce its proper organism.

To state that this organization can be expressed in terms of memory, is simply to state the familiar truth that matter and force are different aspects of the same thing; that all problems of matter may be put into the

terms of force. The statement does not help us at all to picture to ourselves the essential hidden structure of the egg, the organization upon which its wonderful properties depend.

Jäger has recently brought forward an hypothesis which seems at first sight to be a satisfactory epigenesis hypothesis, but examination shows that this too, like Haeckel's perigenesis hypothesis, must be turned into an evolution hypothesis before it can be accepted.

The following extract from his paper ("Zur Pangenesis," von Prof. Dr. C. Jäger. Kosmos iv. 376. 1879) gives, I believe, a fair statement of his views.

"Each organ and tissue of an animal or plant contains, in the molecules of its albumen at least, a specific flavor-and-odor-substance (Duft-und-Würzestoff) which we can easily recognize by our chemical sense, for each organ of an animal has its distinctive flavor. Whenever a full-grown animal experiences hunger, decomposition of albumen takes place in all its organs and tissues, so that their various flavor-and-odor-substances, that is their soul-substance (Sezenstoffe), become free, and penetrate to all parts of the body.

"Now, if there exists in any part of the body protoplasm with the power to attract this substance, this protoplasm acquires in this way its vires formativæ.

"I have already referred with emphasis to the embryological fact that the formation of the reproductive elements takes place at a very early stage in the embryonic life of an animal, and I have designated this as the reservation of germinal protoplasm. As soon as the embryonal cells of the developing animal have become specialized into ontogenitic and phylogenetic cells, the following will occur. Whenever any decomposition of albumen occurs in the developing organism, from hun