were devoted to the description and picturing of such supposed examples of mis-begotten progeny. The belief in the existence of such extraordinary hybrids is still common among so-called "well-educated" people. I have with difficulty avoided causing annoyance and offence to a friend, a celebrated painter, by refusing to admit that a deformed cat, of which he gave me an account, was a hybrid between a cat and a rabbit. A very eminent person whom I was conducting some years ago round the galleries of the Natural History Museum, declared, as we stood in front of the specimen of the Okapi of the Congo Forest, that it was clearly a hybrid between the giraffe and the zebra. He insisted that it was obvious that such was its explanation, and pointed to its striped haunches and legs, and its cloven hoofs and giraffe-like head. I failed to change his opinion. It is the fact–ascertained by careful observation of natural occurrences and by experiment–that, in spite of the almost absolute law or general truth to the effect that the members of a species (whether of plant or animal) only produce fertile offspring by mating with members of that same species, yet there are rare instances known in which individuals of two distinct but allied species have mated and produced fertile offspring. The cases in which such unions have resulted in the production of offspring, but in which the offspring so produced prove to be infertile–that is, incapable of producing offspring in their turn–are much more numerous. An important distinction has also to be made between cases of either fertile or infertile hybrid-production which occur spontaneously in nature, and those in which man by separating the parent animals or plants from their natural conditions of life, or by bringing about impregnation (as in "pollinating" one flower with the pollen-dust of another) succeeds in obtaining a "cross" or "hybrid," whether fertile or infertile, not known to occur in "wild" (that is, not humanly controlled) nature. The rarest case would be that of the production of fertile hybrids in uncontrolled natural conditions. Such possibly occur in the case of some fishes in which the fertilization of the eggs takes place in water, the fertilizing microscopic sperms passing from the males like dust into the water and thus reaching the eggs laid by the females. Occasionally hybrids are thus produced between some common fresh-water fishes–species of the same genus–and between species of flat-fish, such as the turbot and the brill, though it is difficult to be sure that the rare hybrids so produced are fertile even if they attain to maturity. The same is true as to certain small flowering plants having distinct regions of natural distribution and occurrence. At the confines of the regions proper to two such allied species, insects passing from one to the other do sometimes effect a reciprocal fertilization of the two species, and a natural hybrid is the result.