246 GEOGRAPHICAL DISTRIBUTION [pt. ii, ch. xxii 



picked out in the same way, through showing irregularities in 

 regard to their distribution, judged by Age and Area (130, 

 p. 337), and so on. Whenever a species is found whose distribu- 

 tion is markedly different from what one would expect under 

 this hypothesis, that species is nearly always found to be an 

 introduction, or of doubtful identification, or in some way 

 irregular. 



Sufficient has been said in Chapter xviii about the Hollow 

 Curve of Distribution, and both this subject and that of Evolu- 

 tion will be treated of in fuller detail in a later book. It is clear 

 that Age and Area becomes sim]:)ly a corollary of the larger law 

 that was indicated in what was said about Evolution. 



There are many other directions in Avhich Age and Area may 

 prove to be a very useful hypothesis in dealing with problems 

 of distribution, but in the present somewhat controversial stage 

 in which the matter remains, it is better not to attempt too 

 closely to define, or even to outline, new positions. The fact 

 remains that Age and Area (with its subsidiary hypothesis of 

 Size and Space) is strongly supported bj^ very numerous facts 

 which demand an explanation that is largely mechanical, and 

 that the more inasmuch as the same type of facts is exhibited 

 both by animals and by plants. It is also clear that in dealing 

 with questions of Geographical Botany, the statistical method, 

 which has remained almost untouched since Hooker long ago 

 (p. 104) pointed out its usefulness, will probably plaj^ an 

 important part. 



