NO REFLEXIVE SELECTION BETWEEN GROUPS. 131 



2 Isolation Prevents Reflexive Selection between Grcups. 



We are now prepared to understand one reason why isolation result- 

 ing from indiscriminate separation is in time transformed into segre- 

 gation. Isolation is in its very nature the suspension, not only of one 

 form, but of all forms of reflexive selection between the separated portions 

 of the species. The importance of the cessation of natural selection 

 in producing the different stages of the degeneration of organs that 

 have ceased to be of use has been fully discussed by Romanes (see 

 Nature, Vol. 41, p. 437, and previous communications there referred 

 to), who points out that, as the power of the special form of heredity 

 by which any organ has been produced has been built up by many 

 generations of natural selection that have acted in favor of the organ, 

 so the gradual weakening of that power follows the cessation of 

 the natural selection. Professor Weismann seems to appeal to the 

 same principle when he attributes the reduced size of "rudimentary 

 organs" to the action of "panmixia." Now, since isolation always 

 includes the complete cessation of reflexive selection between the 

 separate groups, a similar principle is introduced, and the result must 

 be the weakening of the power of heredity by which the portions of the 

 species were held in correspondence with each other before their sepa- 

 ration. I have elsewhere shown that isolation necessarily disturbs 

 unstable adjustments; and we here see that the most stable of the 

 adjustments by which each part of a species is kept in correspondence 

 with every other part gradually becomes unstable, under the con- 

 tinued influence of isolation. Whenever a species is divided into two 

 portions that do not interbreed, the forms of reflexive selection above 

 described will cease to act between the two portions, and they will 

 continue in sexual, social, physiological, and industrial harmony with 

 each other only in so far as the force of the old heredity holds them to 

 the old standards. But the force of heredity in these respects will in 

 time fail if the reflexive selection that held the original stock in accord 

 is entirely removed in its action between the two portions. If the 

 separate breeding is long continued, incompatibility in all these re- 

 spects tends gradually to arise ; but it is manifest that incompatibility 

 of industrial habits implies diversity in the forms of environal selec- 

 tion shaping each portion. I therefore maintain that separation, 

 which necessarily includes the cessation of reflexive selection between 

 the portions separated, is a cause of segregation and divergence and 

 that it introduces diversity of environal selection, which is a still 

 further cause of divergence. 



