Historically, the theory of evolution in its first manifestation was an extension to the historical sciences generally of a purely biological conception, that of the origin of species as a consequence of the struggle for existence. It was found that much else in the manifold of what is, many other differences between related things, besides the differences which mark off one species of animals from another, might be accounted for, historically, by the theory that those differences were but the sum and the accumulation of an infinite number of small modifications which had given the thing an advantage over its rivals in the struggle for existence. Strictly speaking, all that this remarkable and wide-reaching discovery implied as a matter of logic was that between animals and things not animal there existed an analogy or resemblance, in virtue of which it was logical to argue from things animal to things not animal just so far as the resemblance between them went, but not further. Very naturally, however, it happened that with this originally biological conception all its biological implications were taken over, and it was (and is) argued not merely that there are great and fruitful resemblances between, say, society and an animal organism, but that societies are animal organisms. In fine, sociology was treated as a department of biology. The fallacy that science demonstrates what is, and that what science does not account for has no real existence, thus made its appearance simultaneously with the birth of the evolution theory. The resemblances between the evolution of the social organism and of animal organisms could be accounted for by the biological theory of the struggle for existence; the differences, therefore, must be denied or laboriously explained away. With the growth of sociology, however, it is becoming apparent that the evolution of society has laws, some of which do indeed coincide with those of animal evolution, but others of which are peculiar to sociology in the same sense as the laws of chemistry are distinct from those of physics. Sociology is accordingly revolting from its bondage to biology: the plain fact that society is not an animal is beginning to make itself felt. The resemblances between the organisation of society and that of an animal are freely admitted, but the differences are beginning to claim consideration also; and the sound doctrine is beginning to assert itself that by experience alone, experience of what is, and not by any _a priori_ dogmatism as to what in the name of science must be, can we tell how far the resemblances extend as a matter of fact and where the differences begin. That the evolution theory must be the gainer by thus admitting the facts instead of denying their existence is clear; if sociology is not a branch of biology, and yet the two sciences have certain laws in common, a great step is at once taken towards demonstrating the existence of certain general principles of evolution