The evo

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. lution 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,

although it may be an approximation to the truth, it cannot be regarded as a complete and satisfactory explanation.

The object of this work is to present a new hypothesis which will be seen to bear a close resemblance to the one which has been advocated by Darwin, although careful examination will show that it is in reality very different. I hope to show that it is not open to the objections which are urged against the pangenesis hypothesis, while it contains all the features which give value to the latter.

CHPTER IV.

A NEW THEORY OF HEREDITY.

The objection to the hypothesis of pangenesis would be almost entirely removed if it could be simplified-Statement of a new theory-Heredity is due to the properties of the egg-Each new character has been impressed upon the egg by the transmission of gemmules-Tendency to form gemmules is due to the direct action of external conditions-The ovum is the conservative element-The male cell is the progressive elementThis theory has features of resemblance to most of the hypotheses which have been noticed-It fills most of Mivart's conditions also-It is not necessary to assume that the ovum is as complicated as the adult―There are many race characters which are not congenital-There are many congenital characters which are not hereditary-Direct action of external conditions-Our theory stands midway between Darwin's theory of natural selection and Lamarckianism.

IF the hypothesis of pangenesis could be so remodelled as to demand the transmission of only a few gemmules from the various parts of the body to the reproductive elements, instead of the countless numbers which are demanded by the hypothesis in its original form, we should escape many of the objections which have been urged against it.

If it can be shown that these few gemmules are not necessarily present at all times and in all parts of the body, but only occasionally and in certain regions, we shall escape the difficulty presented by Galton's experiments, and the presumption in favor of the hypothesis will be greatly increased.

If the theory of heredity, in its new form, agrees with