i 4 PROBLEMS OF GENETICS 



declaration might be extended to other orders and other 

 lands. 



We know, of course, that the phenomenon of specific diversity 

 is complicated by local differentiation: that, in general, forms 

 which cannot disperse themselves freely exhibit a multitude of 

 local races, and that of these some are obviously adaptative, 

 and that a few even owe their peculiarity to direct environmental 

 effects. Every systematist also is perfectly aware that in dealing 

 with collections from little explored countries the occurrence 

 of polymorphism or even of sporadic variation may make the 

 practical business of distinguishing the species difficult and 

 perhaps for the time impossible; still, conceding that a great 

 part of the diversity is due to geographical differentiation, and 

 that some is sporadic variation, our experience of our own floras 

 and faunas encourages the belief that if we were thoroughly 

 familiar with these exotic productions it would usually be 

 possible to assign their specific limitations with an approach 

 to certainty. 



For apart from any question of the justice of these wider 

 inferences, if we examine the phenomenon of specificity as it 

 appears in those examples which are nearest to hand, surely we 

 find signs in plenty that specific distinction is no mere consequence 

 of Natural Selection. The strength of that proposition has 

 lain mainly in the appeal to ignorance. Steadily with the growth 

 of knowledge has its cogency diminished, and such a belief 

 could only have been formulated at a time when the facts of 

 variation were unknown. 



In Darwin's time no serious attempt had been made to ex- 

 amine the manifestations of variability. A vast assemblage of 

 miscellaneous facts could formerly be adduced as seemingly 

 comparable illustrations of the phenomenon "Variation." 

 Time has shown this mass of evidence to be capable of analysis. 

 When first promulgated it produced the impression that varia- 

 bility was a phenomenon generally distributed amongst living 

 things in such a way that the specific divisions must be arbitrary. 

 When this variability is sorted out, and is seen to be in part a 

 result of hybridisation, in part a consequence of the persistence 



