This system would require no material variation from the general plan of construction that is common to the different mammals of this class in respect to the parts where the resemblances are kept up throughout the series, such as those of the skeleton, muscles, nerves, viscera, and other organs that are found in all of them. But for the introduction of this peculiar system of reproduction and continuation of the species, there was needful a special and most extraordinary contrivance. If such a contrivance or anything like it had been produced by human skill, and been introduced into a mechanical structure, we should not hesitate to say that there had been an invention of a most special character. When you follow this system through the different animals in which it is found operating, and find that the period of gestation and of suckling is varied for each of them, that for each there is the necessary modification of trunk, situation of the organs, assimilation of food and formation of milk, and many other peculiarities, what are you to conclude but that there has been an adaptation of a new system to a general plan of construction, and that while the latter remains substantially the same, it has had ingrafted upon or incorporated with it a most singular contrivance, so original, comprehensive, and flexible, that its characteristic principle admits of the most exact working in animals that are as far asunder as man and the horse, or as the horse and the seal, or as the seal and the bat?