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 JiypotJicse, 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 



