812 THEORY OF EVOLUTION. 
factor in evolution will vary according to stringency of the 
eliminating process, and it must be noted that the ‘struggle 
for existence” varies in intensity within wide limits, that 
it requires to be investigated for each case, and cannot be 
postulated as a force of nature. 
The importance of the factor will also depend on the 
number, nature, and limits of the variations which occur. 
Thus a new species might arise, either by the occurrence of 
a discontinuous variation of considerable magnitude, or by 
the eliminating process acting for many generations on a 
‘series of minute continuous variations. 
Darwin also believed in the importance of sexual selection, 
in which the females choose the more attractive males, which, 
‘succeeding in reproduction better than their neighbours, 
tend to transmit their qualities to their numerous male 
heirs. But this and other forms of reproductive selection 
es be regarded as special cases of natural selection. 
2. “ Zsolation.” —Under this title, Romanes, Gulick, and 
others include the various ways in which free intercrossing 
is prevented between members of a species, eg. by 
geographical separation, or by a reproductive variation 
‘causing mutual sterility between two sections of a species 
living on a common area. Without some “isolation” 
tending to limit the range of miutual fertility within a 
species, or bringing similar variations to breed together, a 
new variation is liable, they say, to be “swamped” by 
intercrossing. But definite facts as to this “swamping,” 
and in many cases as to the alleged “isolation,” are hard to 
find, nor can we say that a strong variation will not persist 
unless it be ‘‘isolated.” In fact, much evidence has been 
gathered in recent years which shows that certain kinds of 
variations are very strongly heritable and do anything. but 
“blend.” Romanes’ view, however, was that “without 
isolation, or the prevention of free intercrossing, organic 
evolution is in no case possible. Isolation has been the 
universal condition of modification. Heredity and varia- 
bility being given, the whole theory of organic evolution 
becomes a theory of the causes and conditions which lead 
to isolation.” It must be admitted that some forms of 
isolation lead to inbreeding, and this to “ prepotency,” 
which often implies the persistence of individual variations. 
