What, then, is the indirect and collateral evidence? It consists, as we have already seen, of two principal classes of phenomena: first, resemblances of fœtal development which are found on comparing the fœtal growth of different species of animals; second, resemblances in the structure of different species of animals after birth and maturity. These various resemblances are supposed to constitute proof of descent from a common stock, which may be carried back in the series as far as the resemblance can be carried, at whatever point that may be. Thus, in comparing all the vertebrata, we find certain marked peculiarities of structure common to the whole class: the deduction is, that all the vertebrate animals came from a common stock. In comparing all the mammalia, we find certain marked peculiarities of structure common to the whole class: the deduction is that all the mammalia came from a common stock. Going still further back in the supposed series, we come to the amphibians, as the supposed common stock from which the vertebrate and mammalian land animals were derived; and, comparing the different classes of the amphibians, we find certain resemblances which point to the fish inhabitants of the water as their common stock; and then we trace the more highly organized fishes through the more lowly organized back to the aquatic worm, which may itself be supposed to have been developed out of a single cell.[75] The resemblances of structure, wherever we make the comparison between different species, are referable to an ideal plan of animal construction, followed throughout a class of animals, and adjusted to their peculiar differences which distinguish one species from another, just as in the vegetable world there is an ideal plan of construction of trees followed throughout a class of plants, and adjusted to the peculiar differences which distinguish one kind of tree from another. As between man and the monkey, or between man and the horse, or the seal, or the bat, or the bird, there are certain resemblances in the structure of the skeleton, which indicate an identity of plan, although varied in its adjustments to the distinguishing structure of each separate species of animal. In a former chapter, I have shown why the adoption of an ideal plan of a general character is consistent with what I have called the "economy of Nature" in the special creation of different species. On a careful revision of the subject, I can see no reason to change the expression, or to modify the idea which it was intended to convey, and which I will here repeat. It is entirely consistent with the conception of an infinite and all-wise creating power, to suppose that in the formation of a large class of organisms, all the constructive power that was needed for the formation of a general plan was exercised throughout the class, and that there was super added the exercise of all the power of variation that was needful to produce distinct species.