Discussion and Summary.
Turning now to a consideration of the general significance of these numerous examples, the question which first presents itself is: Do they really represent the commencement of evolutionary change? In other words: Are they species in the making? A large number of workers (e.g. Schroder, Pictet, Cuénot, Nuttall, Harrison, Adkin and Sandground, to mention only a few) have answered in the affirmative with regard to the particular case with which each was concerned1. It seems worth while now to consider the question as a whole. If these races are indeed nascent species, then we must assume that their characteristics are sooner or later germinally fixed and that there is some degree of isolation, physiological or habitudinal, which ensures their continuance. How much evidence is there for these assumptions?
It seems to the writer that perhaps the most important consideration in this connection is that of isolation. As has long been realised, the great difficulty in regarding mutations or any other inheritable variations produced in small numbers, as the starting point of new species, is that of accounting for the spread of such throughout a population. If the differences which serve to distinguish one species from another were of a kind likely to be of immediate survival value, then the problem would be simpler, but this does not appear to be the case with the general run of specific characters, still less of varietal ones. The importance of geographical isolation in fostering such variations is undoubted, but in many cases it is unlikely that this factor can come into play until a comparatively late stage in the process; the first steps in the spread of a new variant are little assisted by geographical barriers. If, however, there is some form of physiological isolation such as is provided by various small biological differences and by a repugnance for cross‐mating, then the process seems much easier to comprehend. Charles Darwin saw this clearly, as is evinced by his great interest in Bates' statement (Trans. Linn. Soc.23, 501): “The process of the creation of a new species I believe to be accelerated in the Ithomiae and allied genera by the strong tendency of insects, when pairing, to select none but their exact counterparts.” As Carpenter (1913, Proc. Ent. Soc. p. lxxxviii) points out, Darwin (Life and Letters, 2, 392) wrote to Bates regarding this generalisation: “I look upon this fact as very important”; but later rather severely criticised him for making the statement in general terms without sufficient evidence. That the evidence even now is far from being as extensive as could be wished is made clear by Richards (1927, pp. 345–347).
As has already been seen in this review, in the few instances among naturally occurring biological races in which the matter of selective mating has been put to the test, the results have been positive, although not always, it must be said, on a scale large enough to be very convincing. There is here a big field for further work of a very great importance, ...