In relation to the question of the possibility of establishing hybrids between various species experimentally, I must (before going on to the cognate question of "mongrels") tell of an interesting suggestion made to me by my friend Professor Alphonse Milne-Edwards not long before he died, and never published by him. He was director of the Jardin des Plantes in Paris, where there is a menagerie of living beasts as well as a botanic garden and great museum collections and laboratories. He held it to be probable, as many physiologists would agree, that the fertilization of the egg of one species by the sperm of another, even a remotely related one, is ultimately prevented by a chemical incompatibility–chemical in the sense that the highly complex molecular constitution of such bodies as the anti-toxins and serums with which physiologists are beginning to deal is "chemical"–and that all the other and secondary obstacles to fertilization can be overcome or evaded in the course of experiment. He proposed to inject one species by "serums" extracted from the other, in such a way as seemed most likely to bring the chemical state of their reproductive elements into harmony, that is to say, into a condition in which they should not be actively antagonistic but admit of fusion and union. He proposed, by the exchange of living or highly organized fluids (by means of injection or transfusion) between a male and female of separate species, to assimilate the chemical constitution of one to that of the other, and thus possibly so to affect their reproductive elements that the one could tolerate and fertilize the other. The suggestion is not unreasonable, but would require a long series of experiments in which the possibility of producing such "assimilation," even to a small extent and in respect of less complex processes than those ultimately aimed at, would have to be, first of all, established. My friend did not live to commence this investigation, but it is possible that some day we may see the obstacle to the union of ovum and sperm of species, which are to some extent allied, removed in this way by transfusion or injection of important fluids from the one into the other.