[48] _Nature_, vol. xliii. p. 410, and vol. xliv. p. 29. It is in relation to my presentment of the impossibility of natural selection alone causing polytypic evolution, that Mr. Wallace has been at the pains to show how the permission of intercrossing (panmixia) is necessary for natural selection in its work of causing monotypic evolution. And not only has he thus failed to perceive that the "difficulty" which intercrossing raises against the view of natural selection being of itself capable of causing polytypic evolution in no way applies to the case of monotypic; but as regards this "difficulty," where it does apply, he says:-- Professor G. J. Romanes has adduced it as one of the difficulties which can alone be overcome by his theory of physiological selection[49]. [49] _Darwinism_, p. 143. This, however, is a misapprehension. I have by no means represented that the difficulty in question can alone be overcome by this theory. What I have represented is, that it can be overcome by any of the numerous forms of isolation which I named, and of which physiological selection is but one. And although, _where common areas are concerned_, I believe that the physiological form of isolation is the most important form, this is a very different thing from entertaining the supposition which Mr. Wallace here assigns to me. * * * * * I may take this opportunity of correcting a somewhat similar misunderstanding which has been more recently published by Professor W. A. Herdman, of Liverpool; and as the case which he gives is one of considerable interest in itself, I will quote his remarks in extenso. In his _Opening Address to the Liverpool Biological Society_, Professor Herdman said:-- Some of you will doubtless remember that in last year's address, while discussing Dr. Romanes' theory of physiological selection, I quoted Professor Flemming Jenkin's imaginary case of a white man wrecked upon an island inhabited by negroes, given as an illustration of the supposed swamping effect by free intercrossing of a marked variety with the parent species. I then went on to say in criticism of the result at which Jenkin arrived, viz. that the characteristics of the white man would be stamped out by intercrossing with the black:--