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RECAPITULATION AND CONCLUSION.
The obscurity and complexity of the phenomena of heredity afford no ground for the belief that the subject is outside the legitimate province of scientific inquiry. The existence, in a simple and un specialized egg, of the potentiality of a highly organized and delicately adjusted animal, with special functions, instincts and powers of adaptation, with the capacity for establishing and perpetuating harmonious relations to the changing conditions of the world around it, is certainly one of the most profound problems of the material universe.
The fertilized egg is one of the greatest wonders within our knowledge, but this is no reason for refraining from studying it.
If we believe that living things have become what they now are by a process of gradual evolution, and that they owe their characteristics to the influences to which they have been exposed in the past, we must believe that the properties of the egg are capable of explanation, as far as these determining causes are open to study.
If we accept the generalizations of modern science, and hold that an unicellular ovum is homologous with and is descended from a remote ancestral unicellular organism, and that its properties have been gradually acquired by the natural selection of favorable variations, we must believe that the origin of its properties is as much within our reach as the origin of species.
The most prominent characteristic of heredity is that it may be brought about not only by the various forms of asexual reproduction, but also by the sexual union of two reproductive elements, each of which is homologous with the other cells of the body.
In the lower animals and plants the cells which thus unite with each other, or conjugate, are similar in form, and probably in function also; but in all the higher organisms the male cell is very different from the ovum in form, size, and structure, as well as its mode of origin.
The present structure of each organism is the resultant of two factors, which we may call adherence to type and adaptation to new conditions, or if the use of terms without teleological implications is desired, we may speak of them as heredity and variation, or we may follow Haeckel and call them memory of past experiences, and perception of new relations. The precise terms to be used is a matter of little consequence. The essential thing is the recognition of the fact that each organism is the resultant of two factors, and that evolution is two-sided. An animal is what it is because it has the power to hold on to the experiences and adaptations which fitted its parents for their place in nature, and the parents acquired those peculiarities in virtue of their powers to gradually adjust their structure and habits to their environment.
This is the morphological side of evolution. Looking at it from its dynamical or functional side, we notice that each step in the process of advancement has been readied by divergent specialization and by physiological division of labor. Animals diverge from each other by acquiring the power to occupy different fields, to procure and use different kinds of food, to exist in different media, etc., and the organs and tissues and cells of a highly specialized animal or plant are adapted to perform definite, restricted functions exactly and efficiently, while each part of a low organism fills many offices, but fills them all imperfectly.
We find in all except the lowest organisms that heredity is brought about by two dissimilar reproductive elements, and we find that each organism is the resultant of two factors—heredity and variation.
It is natural to inquire whether there may not be some connection between these two relations; whether the natural selection of favorable variations has not acted upon the reproductive elements as it has upon the mature organisms; whether it has not brought about a physiological division of labor between these elements; whether their originally similar functions have not gradually become specialized until one has become the conservative medium, and the other the agent of progress in heredity.
According to the view advocated in this book, such has actually been the history of the evolution of sex, and natural selection has evolved, in all the higher organisms, a secondary law of heredity, which enables it to do its work rapidly and effectively, with little waste.
In the metazoa and in the higher plants, natural selection is not a crude, rough " hit or miss" method of evolution, for the law of heredity, itself a result of the law of natural selection, is that the ovum is the vehicle of heredity, while gemmules or cell-germs from cells in all parts of the body, are transmitted to the ovum by the male cell, thus causing variation when and where it is needed.
This view is opposed to the conclusion of many high authorities that there is no difference in the functions of the sexual elements, but examination shows that the reasons which they have given for this conclusion admit of another simple explanation.
Darwin's reason for his statement that each sexual element has the power to transmit every single characteristic of the parent form, and that it is an error to suppose that the male transmits certain characters and the female other characters, is that when hybrids are paired and bred inter se, the characters of either grandparent often reappear in the progeny.
A little thought willshow that it is impossible to prove any such conclusion in this way. If two animals which differ from each other in every respect could be made to cross, the result would furnish conclusive evidence as to the correctness or incorrectness of Darwin's statement, but in any possible cross the parents are essentially alike, and they differ only in minor features of recent acquisition. The possibility of parthenogenesis proves that the ovum does transmit the entire organization, but it is impossible to show, from the phenomena of crossing, that the male element* has the same power.
The reason given by Huxley for his opinion that an animal inherits every characteristic of each parent, is that the ovum and the male cell are homologous with each other, and that all the cells of the body are descended, by a process of division, from the compound germ which is formed by their union.
Homology, or similarity of origin, is no ground for assuming similarity of function, and the fact that the male cell and the egg are homologous with each other is no reason whatever for a belief that their parts in heredity are alike.
The fact that either sex ...ay, under certain circumstances, acquire the secondary sexual characters of the other, seems at first sight to show that the whole organization of the male exists in a potential and latent state in the body of every female, and that the whole organization of the female is latent in every male; that each individual is a complete double person. If we accept this conclusion it is only logical to conclude that the power to revert or acquire the characteristics of remote ancestors proves the existence, in a latent state, in each individual, of the complete organization of each of a long series of ancestors of both sexes.
This subtle metaphysical conception is so foreign to the methods and tendencies of modern thought, that when we compare it with Hunter's simple and definite statement that the natural history characteristics of any species of animal are to be found in those properties that are common to both sexes, there does not seem to be any room for choice. The view that each individual inherits all the characteristics of the species, and that the distinctive characteristics of the male are arrested in certain ones, while the distinctive features of the female remain latent in others, furnishes a simple and adequate explanation of the facts, and removes all necessity for the subtle, complex and unthinkable, compound personality hypothesis.
In this connection the interesting and practical question, what determines the sex of the embryo, can hardly fail to suggest itself to the reader. I have refrained from a discussion of this important point in the body of this work, as it has no direct bearing upon our argument and I have no solution to offer. As I have so far omitted all reference to the subject, 1 will take occasion now to call attention, in this connection, to the facts detailed on pp. 55-69. The reader will'see that all female bees are born from fertilized eggs, and all male bees from unfertilized eggs; while the unfertilized eggs of daphnia give rise to females only, and in many of the gall wasps both males and females are born from parthonogenetic eggs. There