96 Evolution and Adaptation 



A definite change in the surroundings might bring about 

 a definite change in the next generation, because the new con- 

 dition acts either on the developing organism, or on the egg 

 itself from which the individual develops. The distinction 

 may be one of importance, for, if the new condition only 

 effects the developing organism directly, then, when the in- 

 fluence is removed, there should be a return to the former 

 condition ; but if the egg itself is affected, so that it is 

 fundamentally changed, then the effect might persist even if 

 the animal were returned to its former environment. More 

 important still is Darwin's recognition of the cumulative 

 effect in a given direction of external influences, for a new 

 variation, that was slight at first, might, through prolonged 

 action, continue to become more developed without any other 

 processes affecting the organism. 



From the Darwinian point of view, however, the all-im- 

 portant source for the origin of new forms is the fluctuating 

 variation, which is made use of both in the process of arti- 

 ficial and of natural selection. We may now proceed to 

 inquire how this is supposed to take place. 



It has been stated that, by means of artificial selection, 

 Darwin believes the breeder has produced the greater number 

 of domesticated animals and plants. The most important 

 question is what sort of variations he has made use of in 

 order to produce his result. Has he made use of the 

 i fluctuating variations, or of the definite ones ? It is diffi- 

 cult, if not impossible, to answer this question in most 

 cases, because the breeder does not always distinguish be- 

 tween the two. There can be little question, however, that 

 he may sometimes have made use, of the definite kinds, 

 whether these are the outcome off external or of internal 

 influences. \The question has been seriously raised only in 

 recent years, and we are still uncertain how far we can accu- 

 mulate and fix a variation that is of the fluctuating kind. In 

 a few cases it has been found that the upper limit is soon 



