has thus made two great experiments of the same type, and if one be reasonably careful to avoid arguing from analogy, it is possible to use one case to illuminate the other by furnishing hints as to what mechanisms may be looked for and in what directions inquiry may profitably be pursued.
The sporadic occurrence of gregariousness at widely separated points of the animal field—in man and sheep, in ant and elephant—inclines one to suppose that multicellularity must have arisen also at multiple points, and that the metazoa did not arise from the protozoa by a single line of descent. It {104} suggests also that there is some inherent property in mobile living organisms that makes combination of individuals into larger units a more or less inevitable course of development under certain circumstances and without any gross variation being necessary to initiate it. The complex evolution which multicellularity made possible, and perhaps enforced, can scarcely fail to make one wonder whether the gregarious animal has not entered upon a path which must of necessity lead to increasing complexity and