310 Geographical Distrihiition of Plants 



incident of the Glacial period a whole flora may have moved down and 

 up a mountain side, while only some of its constituent species would 

 be able to take advantage of means of long-distance transport. 



I have dwelt on the importance of what I may call short-distance 

 dispersal as a necessary condition of plant life, because I think it 

 suggests the solution of a difficulty which leads Guppy to a conclusion 

 with which I am unable to agree. But the work which he has done 

 taken as a whole appears to me so admirable that I do so with the 

 utmost respect. He points out, as Bentham had already done, that 

 long-distance dispersal is fortuitous. And being so it cannot have 

 been provided for by previous adaptation. He says^: "It is not 

 conceivable that an organism can be adapted to conditions outside 

 its environment." To this we must agree ; but, it may be asked, do 

 the general means of plant dispersal violate so obvious a principle ? 

 He proceeds : " The great variety of the modes of dispersal of seeds 

 is in itself an indication that the dispersing agencies avail themselves 

 in a hap-hazard fashion of characters and capacities that have been 

 developed in other connections^." "Their utility in these respects is 

 an accident in the plant's life^" He attributes this utility to a 

 "determining agency," an influence which constantly reappears in 

 various shapes in the literature of Evolution and is ultra-scientific 

 in the sense that it bars the way to the search for material causes. 

 He goes so far as to doubt whether fleshy fruits are an adaptation for 

 the dispersal of their contained seeds*. Writing as I am from a 

 hillside which is covered by hawthorn bushes sown by birds, I confess 

 I can feel little doubt on the subject myself. The essential fact 

 which Guppy brings out is that long-distance unlike short-distance 

 dispersal is not universal and purposeful, but selective and in that 

 sense accidental. But it is not difficult to see how under favouring 

 conditions one must merge into the other. 



Guppy has raised one novel point which can only be briefly 

 referred to but which is of extreme interest. There are grounds for 

 thinking that flowers and insects have mutually reacted upon one 

 another in their evolution. Guppy suggests that something of the 

 same kind may be true of birds. I must content myself with the 

 quotation of a single sentence. "With the secular drying of the 

 globe and the consequent differentiation of climate is to be connected 

 the suspension to a great extent of the agency of birds as plant 

 dispersers in later ages, not only in the Pacific Islands but all over 

 the tropics. The changes of climate, birds and plants have gone on 

 together, the range of the bird being controlled by the climate, and 

 the distribution of the plant being largely dependent on the bird*." 



^ Guppy, op. cit. II. p, 99. » Loc. cit. p. 102. ^ Loc. cit. p. 100. 



* Loc. cit. p. 102. » Loc. cit. ii. p. 221, 



