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 mcdusæ. 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 medusa 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 fusion— Reversion 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

of the same species, and the study of crosses and hybrids is therefore a means of separating, to some extent, the influence of one parent from the influence of the other. This is true, however, only with reference to characteristics which are of recent acquisition, for the greater part of the history of two allied species has been the same, and they show in common everything except what has been acquired by each one since they diverged from their common ancestor.

Crossing gives no way of showing whether these common characteristics are or are not transmitted by one parent or the other or by both, but it does give us this information regarding characteristics which appear in one species but not in the other, and it is therefore the best means at our disposal for studying the influence of each parent upon the offspring.

Crossing as a Cause of Variation.

According to our theory of heredity, we can easily see how the crossing of two species or varieties should lead to variability, for when two species or varieties are crossed certain cells of the body will be hybrids between the gemmules of the male parent and the ovarian particles inherited through the female from the egg of the preceding generation. Now the ovarian particle transmits the properties of a cell like that of the female parent, while the gemmule transmits those of a corresponding cell in the father. It is plain that corresponding cells of a female of one species or variety and of a male of another species or variety must be more different from each other than corresponding cells in a male and female of the same species or variety. The hybrid cell formed by their union would, therefore, be expected to differ more from each of them, that is, to vary more than it

does in the offspring of parents of the same variety. It is well known that this is the case; that, in domesticated animals and plants at least, crossing is a great causeaccording to some older writers the only cause-of variation.

Darwin says that it is probable that the crossing of two forms when one or both have long been domesticated or cultivated, adds to the variability of the offspring, independently of the commingling of the characters derived from the two parent forms. He believes that new characters arise in this way in hybrids between domesticated forms, forms which have been rendered variable through cultivation, but he doubts whether we have, at present, sufficient evidence to prove that the crossing of species which have never been cultivated leads to the appearance of new characters.

The following illustrations of this law are quoted from his Variation (Vol. ii. p. 319):

"Gärtner declares, and his experience is of the highest value on such a point, that when he crossed native plants which had not been cultivated, he never once saw in the offspring any new character; but that from the odd manner in which the characters derived from the parents were combined, they sometimes appeared as if new. When, on the other hand, he crossed cultivated plants, he admits that new characters occasionally appeared. . . . According to Kölreuter, hybrids in the genus Mirabilis vary almost infinitely, and he describes new and singular characters in the form of the seeds, in the colors of the anthers, in the colyledons being of immense size, in new and highly peculiar odors, in the flowers expanding early in the season, and in their closing at night. With respect to one lot of these hybrids he remarks that they presented characters exactly the reverse