246 APPENDIX III—LETTERS PUBLISHED IN NATURE. 
others maintain it to be absolutely essential. This latter view has 
arisen from an exaggerated opinion as to the power of intercrossing to 
keep down any variety or incipient species and merge it in the parent 
stock.’’ (Darwinism, p. 144.) 
I think we shall reach a more consistent and complete apprehension 
of the subject by starting with the fundamental laws of heredity, and 
refusing to admit any assumption that is opposed to these principles, 
till sufficient reasons have been given. Laws which have been estab- 
lished by thousands of years of experiment in domesticating plants 
and animals should be, it seems to me, consistently applied to the gen- 
eral theory of evolution. For example, if in the case of domesticated 
animals, “‘it is only by isolation and pure breeding that any specially 
desired qualities can be increased by selection”’ (see Darwinism, p. 99), 
why is not the same condition equally essential in the formation of 
natural varieties and species? If in our experiments we find that 
careful selection of divergent variations of one stock does not result in 
increasingly divergent varieties wnless free crossing between the varteties 
is prevented, why should it be considered an exaggeration to hold that 
in wild species ‘‘the power of intercrossing to keep down any variety 
or incipient species, and merge it in the parent stock,”’ is the same that 
we have found in domestic species. Experience shows that segrega- 
tion, which vs the bringing of ltke to like in groups that are prevented from 
crossing, is the fundamental principle in the divergence of the various 
forms of a given stock, rather than selection, which 1s like to like through 
the prevention of certain forms from propagating; and I think we intro- 
duce confusion, perplexity, and a network of inconsistencies into our 
exposition of the subject whenever we assume that the latter is the 
fundamental factor, and especially when we assume that it can produce 
divergence without the codperation of any cause of segregation divid- 
ing the forms that propagate into two or more groups of similars, or 
when we assume that segregation and divergence can not be produced 
without the aid of diverse forms of selection in the different groups. 
The theory of divergence through segregation states the principle 
through which natural selection becomes a factor promoting some- 
times the stability and sometimes the transformation of types, but 
never producing divergent transformation except as it codperates with 
some form of isolation in producing segregation; andit maintains that 
whenever variations whose ancestors have freely intergenerated are, 
from any combination of causes, subjected to persistent and cumula- 
tive forms of segregation, divergence more or less pronounced must be 
the result. The laws of heredity on which this principle rests may be 
given in the three following statements. 
