A little thought will show that if there were no explanation of the transmission of latent sexual characteristics more simple than the hypothesis of a dual personality, this hypothesis would then be too simple, and would need to be made much more complicated.

The characteristics of the opposite sex are not the only ones which may be latent, and in cases of reversion a parent may transmit to children characteristics which were exhibited by neither parent nor grandparent, and which may have remained latent for many generations.

If we must assume the existence of a dual personality to account for the latent transmission of the characteristics of the grandparent of the opposite sex, we must assume still other personalities to account for reversion to more remote ancestors, and Darwin has not hesitated to carry the hypothesis to this, its logical conclusion.

He says (Variation, ii. 65), "Several authors have maintained that hybrids and mongrels include all the characteristics of both parents, not fused together but merely mingled in different proportions in different parts of the body; or, as Naudin has expressed it, a hybrid is a living mosaic work, in which the eye cannot distinguish the discordant elements, so completely are they intermingled. We can hardly doubt that, in a certain sense, this is true, as when we behold in a hybrid the elements of both species segregating themselves into segment in the same flower or fruit-by a process of selfattraction or self-affinity-this segregation taking place either by seminal or by bud propagation. Naudin further believes that the segregation of two specific elements or essences is eminently liable to occur in the male and female reproductive matter, and he thus explains the almost universal tendency to reversion in successive hybrid generations. But it would, I suspect,

be more correct to say that the elements of both parent species exist in every hybrid in a double state, namely, blended together and completely separated."

In another place (Variation, ii. p. 80) he says: "On the doctrine of reversion, as given in this chapter, the germ becomes a far more marvellous object, for besides the visible changes to which it is subjected, we must believe that it is crowded with invisible characteristics, proper to both sexes, to both the right and left sides of the body, and to a long line of ancestors, male and female, separated by hundreds or even thousands of generations from the present time, and these characters, like those written on paper with invisible ink, all lie ready to be evolved under certain known or unknown conditions."

I shall discuss the phenomena of reversion somewhat at length in another place, and wish to simply call attention at present to the fact that here, as in the case of secondary sexual characters, we have a much simpler explanation in the hypothesis of arrest, and therefore do not need to call in an unknown factor, such as the multiple personality of each individual.

I think that the phenomena of alternation of generations favor this latter supposition even more than the facts of reversion.

The egg-embryo of a hydro-medusa may give rise by budding to an indefinite number of hydroids like itself, and each of these may give rise to other hydroids, and so on indefinitely.

Each one of these may also, under certain conditions, give rise to medusæ quite different from the hydroids and like the original medusæ. As the medusa which are thus produced inherit through a long series of hydra ancestors all the specific characteristics of the origi

nal medusa, we are forced to conclude that each hydroid contains, in a latent state, the power to reproduce a definite specific medusa.

As the hydra and its medusa differ from each other very much more than a male and a female mammal, and have little in common except the general plan of their organization, there seems at first to be no escape from the conclusion that the medusa structure exists side by side with the hydra structure, in each hydroid, as a second personality.

I hope to show, in the chapter on asexual reproduction that alternation of generations is a secondary condition of things, and that it has been brought about by a modification of ordinary metamorphosis.

I think there is every reason to believe that at one time the hydra-larva which hatched from a medusa egg became metamorphosed, by a gradual change during growth, into a medusa.

If this were the case now, there would be no more reason for believing in a hydra personality and a medusa personality than there is for believing that a human child contains a distinct adult personality.

Now we can understand that if such a larva should give rise by budding to other hydroids like itself, they also would have the power to grow into mature medusæ. We can also understand that circumstances might arise to cause the later stages in the development of some of these hydra-larvæ to become latent. We should then have two generations-hydroids without a medusa stage, and hydroids with a medusa stage.

The suppression of the hydra features of the latter would then give us a generation of medusæ with no hydra stage, giving birth to a generation of hydroids with no medusa stage, and these in turn producing a

generation of meduse with no hydra stage. We should then have a case of alternation like that which is presented by ordinary hydro-medusæ.

Summary of Chapter.

A careful review of the reasons which have induced various authors to believe that either sexual element may transmit any characteristic whatever, leads to the conclusion that its truth is not proven.

It is impossible to prove it by the phenomena of crossing, since the only animals which can be made to cross are essentially alike, and differ only in minor points.

The homology between the ovum and the male cell is no reason for supposing that their functions are similar, and the differences between them should lead us to believe that their functions are not alike.

There is no reason for assuming that each sex transmits its entire organization to the offspring, in order to account for the latent transmission of secondary sexual characteristics, since this transmission can be more simply explained by assuming that each embryo inherits but does not necessarily develop all the characteristics of its species.

Reversion and alternation of generations admit of a similar explanation.

We may therefore conclude that there is and can be no proof that each sexual element transmits all the characteristics of the parent, and that there is no a priori absurdity in the hypothesis that the male and female reproductive elements are unlike in function, and are specialized in different directions.

We can therefore enter without prejudice into an examination of the evidence for this latter view.

CHAPTER VI.

THE EVIDENCE FROM HYBRIDS.

Importance of the subject-It furnishes a means of analyzing or isolating the influence of each sexual element—Hybrids very variable-Hybrids from domesticated races more variable than those from wild races-The descendants of hybrids more variable than the hybrids themselves-The offspring of a male hybrid and the female of a pure species are much more variable than those of a female hybrid and the male of a pure species-These facts inexplicable on any view, except the one here presented -Reciprocal crosses-They differ in fertility and in structure -The difference is exactly what our theory requires-Difficulty in explaining transmission of characters without fusionReversion caused by crossing-Two kinds of reversion-Summary.

THE study of hybrids and crosses is of especial interest to us, since it affords us a means, somewhat imperfect it is true, for recognizing, in the offspring, the structure which it owes to each parent.

In ordinary sexual reproduction between animals or plants of the same race, the parents are almost exactly alike, except for their sexual differences; and as nearly every structural feature of the young is a feature of resemblance to each parent, there can be nothing to show that it is inherited from the one rather than from the other.

When distinct races or species are crossed, the case is somewhat different. It is true that the two parents are still very much alike, for species cannot be made to breed. together at all unless they are very closely related. Still they are more different from each other than individuals