86 Evolution and Adaptation 
least as many individuals of a species as are affected, will 
give the starting-point for the new group. But if the new 
variation arises not directly as a response to some change in 
the surroundings, then it might appear in one or in a few 
individuals at a time. Let us consider what the results 
might be under these two heads. 
If amongst the descendants of a single individual a new 
form or a number of new forms were to arise, then, if they 
represented only a variety, they would cross with the other 
forms like the parent species; and, under these conditions, it 
is generally assumed that the new variety would be swamped. 
If, however, the new forms have the value of new species, 
then, ex hypothese, they are no longer fertile with the original 
forms, and might perpetuate themselves by self-fertilization, 
as would be possible in some of the higher plants, and in 
those animals that are bisexual. But as a rule even bisexual 
forms are not self-fertilized, and, therefore, unless a number 
of offspring arose from the same form the chance of propaga- 
tion would be small. 
If, however, a number of new forms appeared at the same 
time and left a number of descendants, then the probability 
that the new group might perpetuate itself is greater, and the 
chance that such a group would arise is in proportion to the 
number of individuals that varied in the same direction simul- 
taneously. In this case the new species has not come from 
a single individual or even from a pair of individuals, but from 
a number of individuals that have varied more or less in the 
same direction. 
This point of view puts the descent theory in a somewhat 
unforeseen light, for we cannot assume in such a case that the 
similarities of the members of even the same species are due 
to direct descent from an original ancestor, because there are 
supposed to have been a number of ancestors that have 
all changed in the same direction. The question is further 
complicated by the fact that the new individuals begin to 
