3 o 4 GEOGRAPHICAL DISTRIBUTION [CH. 



migrations, may often have assisted unintentionally in the 

 dispersal of associated aquatics. 



Turning from the detailed question of the modes of dispersal 

 of hydrophytes, to the more general problem of their geographi- 

 cal distribution, we find that these plants furnish certain data 

 bearing on the theories put forward in recent years by Guppy 

 and Willis. The views of these two authors, though wholly 

 independent, and in many ways quite distinct, seem in some 

 respects to supplement one another. 



The nature of Guppy's hypothesis which he names the 

 Differentiation Theory 1 may be briefly indicated as follows. 

 He supposes that the history of our present flora is " essentially 

 the history of the differentiation of primitive world-ranging 

 generalised types in response to the differentiation of their con- 

 ditions." He expressly points out that his view does not attempt 

 to explain the origin of these primitive generalised families, 

 and he is careful to note that the present distribution is also 

 "an expression of the influence of the arrangement of the con- 

 tinents during secular fluctuations of climate." For lack of 

 space it is impossible here to do justice to Guppy's theory, but 

 we may consider two cases among water plants, to each of which 

 he draws attention as illustrating differentiation and distri- 

 bution within a single genus. One of these is the genus Natas 2 , 

 which he treats in the light of Rendle's monograph 3 . Guppy 

 considers that the polymorphic Naias marina, which occurs 

 almost all over the whole area of the genus, is the primitive type, 

 representing the stock from which the other species are derived. 

 None of the remaining species are so widely distributed, and 

 though some of them have a considerable range, others are 

 extremely localised. In Limnanthemum 21 ^ again, Guppy re- 

 gards nearly all the tropical species as reducible to varieties 

 of L. indicum, which he takes to be another typical poly- 

 morphic species of wide range; it has played a role, in the 

 warm fresh waters of the globe, comparable with that of 



1 Guppy, H. B, (1917), etc. 2 Guppy, H. B. (1906). 



3 Rendle, A. B. (1901). 



