Under these circumstances we do not hesi tate any longer to coincide with Kiilliker and Bischoff* (the latter changed his opinion only recently) " that it is the spermatozoa which, by their contact, fructify the ovum." How this is done remains as much an enigma as the real essence, the remote cause, of every thing else that is done. We are certainly able to watch growing life in its first commencement, to fathom the laws of the successive phases of its developernent ; but the internal relation of all these processes is hidden from our percep tion.
It is possible, and, indeed, even probable, that the material constitution of the sperma tozoa is somehow concerned in fecundation. Whether, however, as Bischoff supposes, the act of impregnation merely takes place accord ing to the laws of the so-termed catalytic power, that a certain internal motion is trans ferred from the spermatozoa to the molecules of the ova, which till then were in a dormant state, we do not venture to decide. At all events, the circumstance, that it is not the spermatozoa of every animal which are capable without any distinction of fructifying every egg, is sufficient in itself to prove that we have not here to deal with such very simple relations. It is an established fact, that only animals of the same species enter voluntarily into sexual connexion, and produce prolific young ones. The importance of this law, for the preservation of once created definite forms of life, is evident.
Exceptions to this law are but rarely found, and generally are due to the interfer ence of man.— Animals of a different species scarcely ever enter into sexual connexion in their natural state ; and, indeed, this act, when it does take place under such circum stances, remains generally, without any conse quences. Fecundation only takes place when
the respective individuals approximate to wards each other in point of genus, and even then the hybrids produced are generally un fruitful. A fructifying act of procreation is known in them only in very rare cases, and that usually only when it takes place with one of the original stock, not among them selves.
This infertility or barrenness of the hy brids, coincides in a very interesting manner with an imperfect developement of the sper matozoa, a relation which we might certainly at once infer from the functional significance of these formations. In many cases there does not even seem to be any production of spermatozoa; a fact proved by the older state ments of Bonnet and Gleichen, as well as by the more recent researches of Prevost and Dumas*, as well as of Hausmann t, with regard to the mule. One of us found the same in the hybrids of goldfinches and canary-birds. In others, real spermatozoa develop themselves ; but they remain smaller than in the stock species (-71,7"/--a-,"'), and vvithout the characteristic cork-screw spirals. The thicker end is generally oblong, and fre quently curved at the point, or of an irregular club form. In addition to this, the sper matozoa of the hybrids do not group together in bundles, owing perhaps to their being usually only small in number, even in the in terior of the separate cysts. The microscopical examination of the semen in hybrids, the ca pacity of propagation of which has been con firmed, would be of importance. It is very probable that the spermatozoa in these cases have a regular developement, and their usual form.