NO REFLEXIVE SELECTION BETWEEN GROUPS. 131 
2 Isolation Prevents Reflexive Selection between Grcups. 
Weare 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 
jorm, 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 
saine 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. Whenevera 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. 
