FOE DARWIN 375 



tion has no scientific standing. The great law of causation is not 

 abrogated, but the outcome is only the result of a large number of 

 small influences whose effects depend on the nature of the material and 

 on the nature of the conditions. It is so important that this fact be 

 clearly understood that I may be pardoned if I call to mind some 

 familiar illustrations. No two leaves on a tree are identical, yet if 

 many are measured, they give the curve of probability. Men are of 

 different heights, yet they range about a mode. Color appears in 

 various shades, yet if standardized, it is found to follow the same laws 

 of chance variation. 



What value have these facts for the theory of evolution? If in 

 every generation we find that the same kinds of individuals recur, the 

 results mean stability, not progress. That this state of affairs actually 

 exists in many species living under the same environment during suc- 

 cessive generations there can be little doubt. But change the environ- 

 ment and the results also change. Another factor comes to light that 

 is independent of outside conditions. It is what has been called prefer- 

 ential mating. If within a group the males and females of certain 

 kinds tend more often to pair with each other, the collective group 

 becomes modified in one or more directions. In man this factor assumes 

 a special importance, for, as Pearson has shown, there is measurable 

 evidence that such mating occurs. 



It has often been urged, and I think with much justification, that 

 the selection of individual, or fluctuating variations could never pro- 

 duce anything new, since they never transgress the limits of their 

 species, even after the most rigorous selection — at least the best evidence 

 that we have at present seems to point in this direction. But a new 

 situation has arisen. There are variations within the limits of Linnean 

 species that are definite and inherited, and there is more than a sus- 

 picion that by their presence the possibility is assured of further definite 

 variation in the same direction which may further and further tran- 

 scend the limits of the first steps. If this point can be established 

 beyond dispute, we shall have met one of the most serious criticisms of 

 the theory of natural selection. 



It is not without interest to note in this connection that Darwin 

 often assumed that fluctuating variations are transmitted to the 

 offspring. The idea that they are not was a later development — the 

 result, it is true, of a better knowledge of the law of fortuitous effects, 

 or of probability. But we have discovered the additional fact that some 

 small variations are inherited. Let us call these definite variations, and 

 if these be the material with which evolution is concerned, Darwin's 

 assumption in regard to the nature of variation will be, in part, justified. 



These small, definite variations appear to be closely allied to those 

 larger, more visible definite variations that we now call mutations. 



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