KOSMICOS. Take care that you have an accurate idea of what the theory of evolution is. Apply it, for example, to the origin of man, as an animal, proceeding "by a series of forms graduating insensibly from some ape-like creature to man as he now exists." This expresses the whole theory as applied to one animal, man, without going behind his ape-like progenitors. It does not suppose a crossing between the ape-like creature and some other creature that was not an ape. It supposes a gradual development of the ape-like creature into the man as he now exists; and, of course, the interbreeding took place between the males and the females of that ape-like race and their descendants--the descendants, through a long series of forms, being gradually modified into men, by the operation of the laws of natural and sexual selection, which I need not again explain to you. SOPHEREUS. Very well, I have always so understood the theory. But then I have also understood it to be a part of the same theory that there is important auxiliary proof of the supposed process of evolution to be derived from what is known to take place in the interbreeding of different races or families of the same animal. Whatever value there may be in this last fact, as auxiliary evidence of the supposed process of evolution, there must have been a time, in the development of the long series of forms proceeding from the ape-like progenitor, when an animal had been produced which could propagate nothing but its own type, and between which and the surrounding other animals no propagation could take place, if we are to judge by what all nature teaches us. You may say that the laws of natural and sexual selection would still go on operating among the numerous individuals of this animal which had become in itself a completed product, and that to their descendants would be transmitted newly acquired organs and powers, new habits of life, and all else that natural and sexual selection can be imagined to have brought about. But at some time, somewhere in the series, you reach an animal of a distinct character, in which natural and sexual selection have done all that they can do; in which there can be no propagation of offspring but those of a distinct and peculiar type, and the invincible barrier against a sexual union with any other type becomes established. For this reason, we must recognize the limits of possible interbreeding. It is best for us, therefore, to come to some understanding of the sense in which we shall use the term "species." For I shall press upon you this consideration--that animals differ absolutely from each other; that there can be no interbreeding between animals which so differ; and yet that, without interbreeding between animals having distinct organizations, natural and sexual selection had not the force necessary to produce, in any length of time, such a being as man out of such a being as the ape.