ISOLATION AND THE ORIGIN OF SPECIES.

O NE OF THE MOST important principles which has emerged from the recent studies of evolution is that the terms "evolution" and "origin of species" are not synonymous, as implied by the title of Darwin's classic. The foundation of this principle is the more precise, objective concept of the nature of species which has been obtained through more careful systematic and particularly cytogenetic studies of interspecific differences and of the barriers between species Chap. VII). The new definitions of the species which have been based on these studies are numerous; a different one can be found in each of the three publications cited in the previous sentence, while the group of essays compiled by Huxley (1940) contains nine more. Comparing these definitions, however, one is struck, not by their diversity, but by the large common ground of agreement between them. All of them stress the importance of genetic and morphological continuity within species, and recognize at the same time that a species may include within its limits an array of morphologically and physiologically diverse genetic types. They also agree that the boundaries between the species of sexually reproducing organisms are real, objective phenomena, and that they are produced by isolating mechanisms which prevent or greatly restrict the exchange of genes between the members of different species. The common ground of agreement between these definitions may be expressed as follows. In sexually reproducing organisms, a species is a system consisting of one or more genetically , morphologically, and physiologically different kinds of organisms which possess an essential continuity maintained by the similarity of genes or the more or less free interchange of genes