Darwin and After Darwin; An Exposition of the Darwinian Theory and a Discussion of Post-Darwinian Questions Volume 3 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1897 edition. Excerpt: ...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. The only difference between Mr. Gulick's essays and my own is, that, on the one hand, he has analyzed much more fully than I have the various forms of isolation; while, on the other hand, I have considered much more fully than he has the particular form of physiological isolation which so frequently obtains between allied species. This particular form of physiological isolation I have called " physiological selection," and claim for it so large a share in the differentiation of specific types as to find in it a satisfactory explanation of the contrast between natural species and artificial varieties in respect of cross-infertility. Mr. Wallace, in his Darwinism, has done good service by enabling all other naturalists clearly to perceive how natural selection alone produces monotypic evolution--namely, through the free-intercrossing of all individuals which have not been eliminated by the isolating process of natural selection itself. For he very lucidly shows how the law of averages must always ensure that in respect of any given specific character, half the individuals living at the same time and place will present the character above, and half below its mean in the population as a whole. Consequently, if it should ever be of advantage to a species that this character should undergo either increase or decrease of it's average size, form, colour, &c, there will always be, in each succeeding generation, a sufficient number of individuals--i. e. half of the whole--which present variations in the required direction, and which will therefore furnish natural...