The Darwinian hypothesis is that this great transition from the one system of reproduction to the other took place between the amphibians and the ancient marsupials, by the operation of the influences of natural and sexual selection. That is to say, the system of reproduction through an egg, which is the characteristic of the amphibians, became changed by gradations and modifications into the system of the lowest mammals, the distinction between the former and the latter being an obvious and palpable one. Then we are to suppose a further change from the marsupials, or the implacental mammals, to that wonderful contrivance, the _placenta_, by which the mother nourishes the fœtus into a more complete state of development before the young animal is born. This enormous change of system is supposed to have been brought about by a struggle among the individuals of one species for food, aided by a struggle between the males of that species for the possession of the females, by the growth and development of organs useful to the animal in the two battles, and by the transmission of these enhanced powers and improved weapons to offspring, and possibly by the crossing of different varieties of the new animals thus produced. But what potency there could be in such causes to bring about this great change it is extremely difficult to imagine, and we must draw largely on our imaginations to reach it. It would seem that if there is any one part of animal economy that is beyond the influence of such causes as the "survival of the fittest," it is the reproductive system, by which the great divisions of the animal kingdom continue their respective forms. Give all the play that you can to the operation of the successful battle for individual life, and to the victory of the best-appointed males over their competitors for the possession of the females, and to the transmission of acquired peculiarities to offspring--when you come to such a change as that between the two systems of reproduction and perpetuation, you have to account for something which needs far more proof of the transitional gradations of structure and habits of life than can now be found between the highest of the amphibians and the lowest of the mammalia. I know not how there could be higher or stronger evidence of design, of a specially planned and intentionally elaborated construction, than is afforded by this great interval between the one reproductive system and the other. But it is time now to pass to those points of resemblance between man and the other mammals which are asserted as the decisive proofs of his and their descent from some pre-existing form, their common progenitor. These points of resemblance may be considered in the following order: