producible by isolation alone (even in a small population) can never be greater than that between the extremes of variation which occur within the whole species at the date of its partition (p. 75). In other words, the possibility of change due to amixia alone is taken to be limited by the range of deviation from the general specific average, as manifested by different individual variations, before the species was divided. Thus the doctrine of amixia fails to recognize the law of Delbœuf, or the _cumulative_ nature of divergence of type when once such divergence begins in a separated section. Therefore, in this all-important--and, indeed, essential--respect, amixia differs entirely from the principle which has been severally stated by Delbœuf, Gulick, and myself. [47] _Loc. cit._, p. 54. Upon the whole, then, we must say that although Professor Weismann was the first to recognize the diversifying influence of merely indiscriminate isolation _per se_ (apogamy), he did so only in part. He failed to distinguish the true essence of the principle, and by overlaying it with a mass of hypothetical speculation, concealed even more of it than he revealed. * * * * * The general theory of Isolation, as independently worked out by Mr. Gulick and myself, has already been so fully explained, that it will here be sufficient merely to enumerate its more distinguishing features. These are, first, drawing the sharpest possible line between evolution as monotypic and polytypic; second, showing that while for the former the peculiar kind of isolation which is presented by natural selection suffices of itself to _transform_ a specific type, in order to work for the latter, or to _branch_ a specific type, natural selection must necessarily be assisted by some other kind of isolation; third, that even in the absence of natural selection, other kinds of isolation may be sufficient to effect specific divergence through independent generation alone; fourth, that, nevertheless, natural selection, where present, will always accelerate the process of divergence; fifth, that monotypic evolution by natural selection depends upon the _presence_ of intercrossing, quite as much as polytypic evolution (whether with or without natural selection) depends upon the _absence_ of it; sixth, that, having regard to the process of evolution throughout all taxonomic divisions of organic nature, we must deem the physiological form of isolation as the most important, with the exception only of natural selection.