CHAPTER II. HISTORY OF THE THEORY OF HEREDITY. Requisites of a theory of heredity.-Historical sketch of speculation on heredity-Evolution hypothesis of Bonnet and Haller-Ovists and spermists-Modern embryological research has shown that it is impossible to accept the evolution hypothesis in its original form-Buffon's speculations upon heredity fails to account for variation-Hypothesis of epigenesis-This hypothesis is. logically incomplete-The analogy between phylogeny and ontogeny gives no real explanation of the properties of the ovum-Haeckel's plastidule hypothesis —This hypothesis is not logically complete unless it involves the idea of evolution-Jager's hypothesis-Ultimate analysis shows that this is at bottom an evolution hypothesis-No hypothesis of epigenesis is satisfactory-No escape from some form of the evolution hypothesis-This conclusion is accepted by Huxley. § 1. Requisites of a theory of heredity. The following list is a brief summary of what seem to me the most important characteristics of the reproductive process in living things: 1. New organisms may be produced by the various forms of asexual generation and from ova. 2. Ova may develop, in certain cases, without fertilization. 3. As a rule the ovum does not develop into a new organism until it has been fertilized by union with a male cell. 4. The ovum and male cell will not unite unless they are derived from organisms with the same or nearly the same systematic affinities. 5. The new organism, whether produced sexually or asexually, is essentially like its ancestors, although it may be quite different from its immediate ancestor, as in cases of alternation. 6. Organisms produced from fertilized ova differ in the following points from those produced asexually: a. As a rule the development of the egg embryo is indirect, and a more or less complicated metamorphosis or alternation of generations must be passed through before the adult form is reached, and the circuitous path thus traversed bears a resemblance to the line of evolution of the species. An organism formed asexually traverses only so much of this path as remains to be traversed by the organism which gives birth to it. b. Reversion, or the appearance of characteristics not exhibited by the parents, but inherited from remote ancestors, is not at all unusual in egg embryos, but it is more rare in those produced asexually. c. New variations, or features which are not inherited, appear continually in organisms produced from fertilized ova, and they may be transmitted either sexually or asexually to future generations, thus becoming established as hereditary race-characteristics. Hereditary variations are extremely rare in organisms produced asexually. 7. The ovum and the male cell are homologous with each other, and are morphologically equivalent to the other cells of the organism. We must therefore believe that their distinctive properties have been gradually acquired, and that their specialization has been brought about by the action of the same laws as those in accordance with which the other specializations of the organism have been produced. 8. Changed conditions do not act directly, but they cause subsequent generations to vary. 9. In the higher animals, where the sexes have long been separated the male is more variable than the female. 10. The result of crossing is not the same when crosses are made reciprocally. 11. The sex of the parent-species affects the degree of variability of hybrids; and when a hybrid is used as the father, and either one of the pure parent-species, or a third species, as the mother, the offspring are more variable than when the same hybrid is used as the mother and either pure parent-species or the same third species as the father. There may perhaps be other requisites which should be included in this list, but I think there can be no doubt that a theory of heredity must recognize and be in harmony with all which are here given. § 2. A sketch of the history of speculation on the theory of heredity. The laborious researches of the students of the science of embryology have yielded a rich harvest of valuable facts, and we now know that the process of cell division by which an unspecialized unicellular egg becomes converted into a many-celled, highly-specialized organism bears the closest resemblance to the process of growth or of ordinary cell-multiplication. We know that all the various forms of reproduction, cell-multiplication, fission, gemmation, conjugation, sexual reproduction, and parthenogenesis, are inter-related in such a way that we must believe that they are different manifestations of the same power, and that they have been evolved one from the other. We know that direct development, metamorphosis, and alternation of generations are not separated from each other by any hard and fast line, and we know too that the changes through which the embryo passes on its road from the egg to maturity show a wonderful parallelism to the series of changes through which the organism has passed during the history of its evolution from lower forms. These results are well worth the labor they have cost, and they illustrate, more clearly than any other facts in biology, the common nature of all living things. They do not, however, contribute directly to a clearer insight into the laws of heredity. Here we are still compelled to go beyond the visible phenomena, and to attempt by the scientific use of the imagination to discover the as yet unseen relations which bind them together. As we enter upon this subject it will be well to bear in mind the wide difference between the end we have in view the discovery of the secondary laws of heredityand the attempt to understand its ultimate cause. The power to reproduce itself, to impress upon dead inorganic matter its own distinctive properties, is one of the fundamental characteristics of living matter; and while we may hope that increase of knowledge may some day enable us to trace the origin of this power, such an attempt forms no part of our present undertaking. We shall accept without explanation the fact that living matter does thus reproduce itself, and we shall confine ourselves to the attempt to discover why the egg of a star-fish for instance, reproduces a star-fish, and the egg of a bee a bee; to discover the origin of the differences between the various forms of reproduction, rather than the cause of what they have in common. The phenomena of heredity in the higher animals, as well as the mechanism of ova and male cells through which these phenomena are manifested, have certainly been produced by slow modification, through the influence of conditions which are to a great extent open to study. The attempt to trace their origin and significance is not a pure speculation, but a legitimate exercise for the scientific intellect. As we should expect from the fascinating nature of the subject, there has been no lack of speculation in the past, and various hypotheses have been proposed from time to time to account for the phenomena of heredity. These hypotheses differ greatly among themselves, but they may beroughly classed as epigenesis hypotheses, and evolution hypothesis: the word evolution being here used, of course, in its old sense, as contrasted with epigenesis. The hypothesis of evolution, pure and simple, as advocated by Bonnet and Haller, is that there is contained in the egg or seed or in the male element a perfect but minute organism, and that the subsequent development of the egg is simply the "evolution" or unfolding of this germ. Up to the end of the last century the prevailing opinion was that each egg contains, in a latent or dormant state, a completely formed organism. The fertilization. of the egg was supposed to awaken this dormant germ, to call its latent potential life into activity; and the process of development was regarded as the unfolding and growth of the already fully formed and perfect embryo. The embryo was held to be not produced by, but simply unfolded from the egg, and the act of reproduction was therefore regarded as eduction not production. According to Huxley (Encyc. Brit., Art. Evolution) "Bonnet affirms that before fecundation the hen's egg contains an excessively minute but complete chick, and that fecundation and incubation simply cause this germ to absorb nutritious matters, which are deposited in the interstices of the elementary structure of which the min |