Originative Factors in Evolution. By J. A. Thomson. 439 



tions are on the same platform, and here we may l^o back to 

 Darwin's distinction between " single variations " and " individual 

 variations," though the terms are not felicitous. By " single 

 variations " Darwin meant sports, abrupt changes of notable 

 amount, such as that which gave rise to the copper-beech in the 

 seventeenth century, or to hornless cattle, or to short-legged sheep, 

 or to Angora rabbits, or to fan tail pigeons. They correspond to 

 G-alton's " transilient variations," to Bateson's " discontinuous 

 variations," to De Vries's '" mutations," and the last name should 

 be kept for them. By " individual variations " Darwin meant the 

 minute, ubiquitous peculiarities which distinguish child from 

 parent, brother from brother, cousin from cousin. Though he was- 

 much interested in brusque " sports," it was to minute fluctuations 

 (or individual variations) that he mainly looked for supplying the 

 raw materials of new species. " The more I work," he said, " the 

 more I feel convinced it is by tlie accumulation of such extremely 

 slight variations that new species arise." Some authors have 

 tried to identify Darwin's slight individual variations or fluctua- 

 tions with the somatic modiflcations already referred to, but this 

 is not what Darwin meant, as is plain from such a sentence as 

 this : " If, as I must think, external conditions produce little 

 direct effect, what the devil determines each particular vg^riation." 

 Moreover, fluctuations or minute variations often arise among 

 animals whose conditions of life appear to be quite uniform. On 

 the other hand, what Johanssen calls fluctuations in '' pure lines " 

 of beans are probably slight modifications due to differences in 

 nurture. Little is known in regard to the transmissibility of 

 fluctuations or minute variations in the Darwinian sense, Ijut the 

 recent work of Castle, for instance, shows that it is in some cases 

 demonstrable. 



It is a curious fact that one of the reasons why Darwin 

 attached little importance to sports or mutations was his belief 

 that they would be swamped in inter-crossing. In reality they 

 are highly transmissible. When they come they often come to 

 stay, unless they are pathological on the one hand, or too super- 

 lative, like geniuses, on the other. What is desirable at present 

 is more evidence of the transmissibility of the small fluctuations 

 of germinal origin, a transmissibility which Darwin assumed 

 without question. 



§ 4. Turning our attention now to the problem of the origin of 

 inborn variations, we may perhaps usefully distinguish two levels 

 of difficulty. There are variations and variations. There are some 

 novelties that imply just a little more or a little less of some 

 quality — a slightly longer tail, a slightly denser blackness, a 

 slightly stronger flight-muscle, a slightly weaker eye ; some that 

 involve a disappearance of an entire character, such as hair or 

 horns, tail or pigment ; some that may be described as obvious 



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