CHAPTER XIV 



DIFFERENTIATION 



The author approached the study of plant-distribution through 

 his investigations into the agencies of dispersal, the results of which 

 are given in his book on Plant Dispersal. At first inclined to attach 

 undue importance to these agencies, the effect of his special inquiries 

 concerning littoral plants and insular floras, he came to learn that, 

 however efficacious they might be in stocking islands with their 

 plants, they acquired a diminished significance in continental regions. 

 We are there brought face to face with problems concerned with 

 past changes in the history of climate, with the relations of land and 

 sea in the lapsed geological ages, and with those mysterious revolu- 

 tions in plant-forms that have affected the whole world. It was the 

 behaviour of the polymorphous or highly variable species in the 

 Pacific islands, differentiating as it does in every group, that first 

 drew his attention towards the real meaning of distribution, a matter 

 discussed in the work above named. 



The Differentiation Theory and its Limitations. — The view 

 that the history of our globe, as far as secondary causes are in opera- 

 tion, is essentially the history of the differentiation of primitive 

 world-ranging generalised types in response to the differentiation of 

 their conditions, is far from being a novel one. It has been indepen- 

 dently acquired by a number of investigators; and, indeed, many 

 lines of inquiry affecting the great groups of animals as well as plants 

 converge towards this conclusion. It does not, however, attempt to 

 explain the origin of types, nor does it account for evolutionary 

 progress, processes which are considered to be under the sway of 

 other influences that are not at present within our field of cognition. 

 It is concerned only with the response of organisms to the demands 

 of their environment, and all that seems purposive in the animal and 

 plant worlds, all that is bound up with the great scheme of progres- 

 sive evolution, is viewed much as the old naturalists were wont to 

 regard it. 



The Author's Association with the Theory. — In the last 

 chapter of his Plant Dispersal, which was published in 1906, the 

 theory is referred to in different connections. Thus the loss of the 

 viviparous habit, that is assumed to have been characteristic of 

 primitive plants under uniform climatic conditions, and the conse- 

 quent development of the rest-period of the seed, are ascribed to 

 the differentiation of climate and to the resulting seasonal variation. 

 But attention is especially paid to the concurrent differentiation of 



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