EVOLUTION AND RACIAL CHANGE 221 



admittedly, when wild, the operation of Natural Selection cannot 

 be observed, and amongst which, when domesticated, it is largely 

 in abeyance. Very many, if not all, biologists believe, also, that 

 variations commonly result from the direct action of the environ- 

 ment on the germ-plasm, and found this opinion mainly on 

 experimental evidence that is, on evidence obtained from plants 

 and animals which have been placed under conditions abnormal 

 to the race and to the individual. But, as already noted, such 

 experiments prove merely that it is sometimes possible to devise 

 conditions in which the germ-plasm, like other living structures, 

 is injured and yet not killed. They do not demonstrate that the 

 variations of a species in its normal environment commonly arise 

 in this way. It is impossible to prove a universal negative, and 

 therefore, if only for that reason, we cannot demonstrate that 

 variations are not so caused. Indeed, I do not maintain anything 

 of the kind. I insist merely, on grounds which I believe are quite 

 unassailable, that the conditions of life are such that it could not 

 persist on earth unless the vast majority of variations were spon- 

 taneous unless the germ-plasm were highly insusceptible to the 

 direct action of the environment. We have already dealt with 

 some of the evidence. But it will be well worth while to consider 

 it more at length, especially as we shall thereby gain an insight 

 into man's recent evolution, and be brought in contact with some 

 practical problems of great importance. 



363. We reached one other conclusion which bears on our 

 present task, namely, that the predominance of retrogressive 

 variations is less marked in the case of old-established than of 

 more recently evolved characters. This, also, is an adaptation. 

 The circumstance that continued selection tends to render 

 characters more stable carries the advantage that ultimately a 

 minimum of selection suffices to maintain a useful trait. The 

 mortality resulting from selection is thus diminished, and the 

 selection (and therefore the progression) of other useful characters is 

 rendered possible without the death-rate exceeding the birth-rate. 



364. Before discussing human heredity, it is necessary to 

 define carefully the meanings we attach to such terms as ' evolu- 

 tion,' 'racial change,' and the like. They have both been used, 

 especially by popular writers, to indicate two quite separate sets 

 of phenomena, which, if we wish to avoid endless confusion, must 

 be clearly distinguished. On the one hand are germinal or truly 

 innate changes; on the other are changes due to a differential 

 play of stimuli. Thus the colour difference between Scandinavians 



