_Seebohm on Isolation._--Seebohm is so strongly influenced by the difficulty from "the swamping effects of free intercrossing," that he is driven by it to adopt Asa Gray's hypothesis of variations as teleological. Indeed, he goes as far as Wagner, for he maintains that in no case can there be divergence or multiplication of species without isolation. He makes the important statement that "the more the geographical distribution of birds is studied, the more doubtful it seems to be that any species of bird has ever been differentiated without the aid of geographical isolation" (_Charadriidae_, p. 17). If this is true, it makes in favour of physiological selection by showing the paramount importance of the swamping effects of intercrossing, and consequent importance of isolation. But it makes against physiological selection by showing that the geographical form of isolation is sufficient to explain all the cases of specific differentiation in birds. But I must remember that the latter point rests largely on negative inference, and that birds, owing to their highly locomotive habits, are the class of animals where physiological selection is likely to be most handicapped. _Herbert on Hybridization._--Herbert tells us that when he first astonished the Horticultural Society by laying before them the results of his experiments on hybridization, his brother botanists took serious alarm. For it appeared to them that this "intermixture of species would confuse the labours of botanists, and force them to work their way through a wilderness of uncertainty." Therefore he was bluntly told by several of these gentlemen, "I do not thank you for your mules." Now, although naturalists have travelled far and learnt much since those days, it appears to me that a modern evolutionist might still turn to the horticulturist with the same words. For assuredly he has no reason to thank the horticulturist for his mules, until he has found a satisfactory answer to the question why it is that natural species differ so profoundly as regards their capacity for hybridizing. _Advance on Herbert's Position._--- If it be said that all my work amounts to showing what Herbert said long ago--viz. that the only true or natural distinction between organic types is the sexual distinction--I answer that my work does much more than this. For it shows that the principle of sterility is the main condition to the differentiation, not merely of species and genera, but also to the evolution of adaptations everywhere, in higher as well as in lower taxonomic divisions. Moreover, even though naturalists were everywhere to consent to abandon specific designations, and, as Herbert advises, to "entrench themselves behind genera," there would still remain the facts of what are now called specific differences (of the secondary or morphological kind), and by whatever name these are called, they alike demand explanation at the hands of the evolutionist.