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esis," we certainly should look for the occurrence of parthenogenesis in ripe ova which have not extruded these bodies.

However this may be, the correctness or incorrectness of the polar-cell hypothesis has no bearing upon our present argument, for the phenomena of parthenogenesis show beyond question that an egg may develop without union with a male cell, and there is no evidence whatever that a male cell ever acts in a similar way.

Other reasons for believing that the ovum and the male cell perform different functions in heredity.

Even if the possibility of parthenogenesis did not show us that the part played in heredity by the ovum is different from that played by the male cell, there are many other reasons for believing that the difference in the form of the two sexual elements corresponds to some profound difference of function.

I shall devote several chapters of this book to the extended discussion and proof of the facts which drive us to this conclusion, and I shall show that the belief in the essential similarity of the functions of the reproductive elements cannot possibly be retained.

When the male of one species or variety is crossed with the female of another species or variety, the hybrid offspring is often very different from that which is produced when the female of the first species is crossed with the male of the second. If the function of the ovum is the same as that of the male cell, we should have exactly the same elements in each case, and should expect the same result. The fact that the result is not the same proves that the elements are not the same either.

In many cases the male of one species will breed

freely with the female of the second species, while absolute sterility follows the union of a male of the second species with a female of the first species. The offspring of a male hybrid and the female of a pure species is much more variable than the offspring of a female hybrid and the male of a pure species. These facts are absolutely inexplicable, if the two sexual elements play similar parts in heredity.

A structure which is more developed or of more functional importance in the male parent than it is in the female parent is very much more apt to vary in the offspring than a part which is more developed or more important in the mother than it is in the father.

These facts, and many others which will be mentioned farther on, compel us to believe that there is some profound functional difference between the ovum and the male cell.

It is, therefore, only reasonable to distrust the absolute correctness and completeness of any hypothesis of heredity, which, like Darwin's Pangenesis hypothesis, recognizes no such difference.

Summary of last two Chapters.

The phenomena of heredity are certainly among the greatest marvels of the material universe, but there is no reason to believe that they lie outside the province of legitimate scientific inquiry. Our present purpose is not to trace them back to their origin or to show that they result from the properties of matter, but simply accepting them as vital phenomena, to trace the secondary laws to which their present form is due. The fact that the distinctive properties of the egg of any living species have been gradually acquired during the evolution of the race through the action of influences which

are, to a certain extent, open to observation and study, gives us ground for believing that we may hope to discover what it is in the structure of the egg, which renders these properties possible. There have been many attempts to do this, but it is impossible to accept any hypothesis which has ever been advanced. The evolution hypothesis, as advocated by Bonnet and Haller, is directly contradicted by the discoveries in the modern science of embryology, and it is accordingly now regarded as having only an historical interest, but the modern epigenesis hypothesis is no more satisfactory, for the resemblance between the evolution of a species from an unicellular ancestor and the development of an individual animal from an unicellular egg is only an analogy.

The efficient cause in the first case, the slow modification of the race by the natural selection of the most favorable variations, is absent in the second case, and there is nothing whatever to take its place. The parallelism between embryology, or the ontogenetic development of the individual, and phylogeny, or the evolution of the race, is one of the most remarkable and instructive generalizations of modern science, and the very existence of the parallelism gives us every reason to hope that an explanation of heredity or of ontogenetic development may be discovered: but to point out the parallelism is, in no sense whatever, to explain heredity.

If the conclusion be true which is accepted by most of the modern advocates of epigenesis, the conclusion that the egg which is to become a man differs in no essential particular from the egg which is to become a starfish, heredity is an insoluble mystery, for we neither possess nor have any grounds for believing that we ever shall possess any knowledge of forces competent to pro

duce from two essentially similar eggs adult animals which are so essentially dissimilar. We cannot attribute this result to natural selection, for this law can only act on successive individuals; we cannot attribute it to the direct action of external conditions, for we know that eggs may give rise to very different animals when placed under identical surrounding conditions. Haeckel's statement that heredity is memory, contains a profound truth, as we have already seen, but it does not help us to understand heredity.

We know memory only in connection with organization, and if we believe that an egg contains the memory of all the past experience of the race, we must believe that it contains a complex organization to correspond to the complexity of this past experience.

So far as Haeckel's hypothesis of perigenesis has any claim to be considered an explanation of heredity, it is an hypothesis of evolution, not of epigenesis.

Jäger's view that the ovum is at first unspecialized, and that it gradually assimilates from its developing parent all the specializations of the structure of the latter, fails to account for reversion or for the transmission of adult characters by immature parents, and the author is compelled to substitute for it an evolution hypothesis when he comes to treat of reversion.

There is no escape from the conclusion that the ovum of an animal actually contains in some form the potentiality of that particular animal, and Huxley acknowledges that the development of an egg is in essence a process of evolution.

We thus find ourselves driven back from the modern hypothesis of epigenesis to the long abandoned hypothesis of evolution, and we must therefore inquire whether our recent great advances in knowledge of the forces

which have produced the various forms of animal and vegetable life, will guide us nearer to the truth than the speculations of the last century. Bonnet and Haller might fairly assume that each species had been what it is now "from the beginning," but we cannot nowaday make any such assumption, and we must believe that the structure of the germ, like the structure of the adult animal, has been gradually acquired by natural selection.

A modern hypothesis of evolution must therefore be a very different thing from the one which Bonnet furnished, and must account for the slow advancement of the germ from generation to generation.

In Darwin's pangenesis hypothesis we have a provisional explanation based upon the generalizations of modern science. It is a true evolution hypothesis, for Darwin believes that an ovum or a male cell is a wonderfully complex structure, and that it contains gemmules to represent each feature in the organization of the adult. One essential difference between this hypothesis and the original hypothesis of evolution as stated by Bonnet, is that Darwin believes that the ovum contains, not the perfect animal in miniature, but a distinct germ for each distinct cell or structural element of the adult. Darwin's hypothesis recognizes the gradual specialization of the ovum during the evolution of the race, for each cell of the body of the parent may at any time transmit to it new gemmules. Most of the objections to it are based upon its complexity, and on the almost infinite number of gemmules which it requires; but besides these objections we know from Galton's experiments that it is impossible to accept it without modification. We also have, in the fact that the functions of the two sexual elements are not alike, a reason for believing that,

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