272 Evolution and Adaptation 
of men having a mode of 5 feet 9 inches, and the 6-foot men 
were selected in each generation, then in six generations this 
type would be permanently established, and this change 
could be effected in two hundred years.! 
Thus we have exact data as to what will happen on the 
average when blended, fluctuating variations are selected. 
Important as such data must always be to give us accurate 
information as to what will occur if things are left to 
“chance” variations, yet if it should prove true that evolu- 
tion has not been the outcome of chance, then the method is 
entirely useless to determine how evolution has occurred. 
More important than a knowledge of what, according to 
the theory of chances, fluctuating variations will do, will be 
information that would tell us what changes will take place 
in each individual. In this field we may hope to obtain data 
no less quantitative than those of chance variations, but of a 
different kind. A study of some of the results of discon- 
tinuous variation will show my meaning more clearly. 
DISCONTINUOUS VARIATION 
Galton, in his book on “ Natural Inheritance,” points out 
that “the theory of natural selection might dispense with 
a restriction for which it is difficult to see either the need 
or the justification, namely, that the course of evolution al- 
ways proceeds by steps that are severally minute and that 
become effective only through accumulation.” An apparent 
reason, it is suggested, for this common belief “is founded on 
the fact that whenever search is made for intermediate forms 
between widely divergent varieties, whether they are of plants 
or of animals, of weapons or utensils, of customs, religion, or 
language, or of any other product of evolution, a long and 
orderly series can usually be made out, each member of which 
differs in an almost imperceptible degree from the adjacent 
1 Quoted from Pearson’s “Grammar of Science.” 
