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a complete explanation of the origin of species, and that it is exposed to certain very serious difficulties.
Still he concludes that the theory is supported by such a mass of evidence that we may fairly believe that our own knowledge, not natural selection, is at fault, and that further research will remove the difficulties by the discovery of other laws.
Naturalists all over the world have acknowledged the justice of this claim, and some, less candid and broadminded than Darwin, seem to have even lost sight of the difficulties.
Now natural selection can act only by the preservation of such variations as chance to appear, and until we know the laws which govern the appearance of variations it must be impossible to decide how far the course of or ganic evolution has been determined by these unknown laws, and how far by natural selection.
We may therefore entertain a reasonable hope, that when the true theory of heredity is discovered, it will, by revealing to us the laws and causes of variation, place the law of natural selection upon a firmer basis, and show that its apparent difficulties are simply due to the narrow limits of our knowledge.
With this introduction I will pass to the discussion of our subject, the nature of heredity.
The attempt to generalize from the whole field of natural science is beset with many difficulties, since the field is so vast that an attempt to give in advance a statement of all the facts upon which reasoning is based would simply confuse the mind of the reader, and burden him with a mass of detail.
It seems best then to start with the generalizations which are believed to bind the facts together, so that the reader may then approach the specific proofs with more interest. The latter method is open to objection, since the reader may be called upon to listen to views which are opposed by accepted authorities, and to wait until the proofs are presented in due course.
I must therefore request the reader to suspend judgment, and to lay aside established opinions, until he has examined the subject upon all sides.
The examination of the history of the subject will furnish an introduction to its scientific discussion, and I have therefore adopted the following plan:
I shall give, first, an outline of the chief hypotheses which have been proposed, from time to time, as an explanation of heredity, with reasons for rejecting them. I shall then present briefly, in outline, a statement of what I believe to be the true explanation. I shall then try to show that this theory furnishes a basis or foundation for the theory of natural selection, and removes the most serious difficulties which have been urged against the latter theory. I shall then show that there is no a priori reason for rejecting the theory of heredity; and that it furnishes an explanation of many well-known facts which cannot without it be seen in their true relations. I shall then attempt to show that it is supported by direct proof, and finally I shall give a statement of the theory in a more extended form.
HISTORY OF THE THEORY OF HEREDITY)
Eequisites 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. Eequisites 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-specks 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 Ave 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