These few instances, which might be indefinitely multiplied, may suffice to make it manifest that organic forms are to be ascribed to causes essentially the same as those which regulate the forms of inorganic bodies: in short, to the laws which force obeys wherever it is found. The peculiar structure which living bodies assume is due to the mechanical conditions under which they are placed, and not to a peculiar power operating to that special end. That peculiar power is, indeed, disproved, if further disproof were needed, by the existence of monstrosities and deformities, in which the end is not attained. The case is like that of the old doctrine that nature abhorred a vacuum. It was found that this was true only to a certain extent, and to varying degrees; just so does the special formative power supposed in living bodies produce peculiar forms only to a limited and varying degree of accuracy.

A word may be said here, also, respecting the doctrine of “types,” or standards, to which all living forms are referred. As a guide to the investigation of the organic world, this idea has proved itself invaluable; and the doctrine of corresponding parts in different organisms, to which it has been made subservient, constitutes, and must continue to constitute, a beautiful branch of physiological science. But it is hardly necessary to say that no formative power is to be ascribed to those types or standards.