DIFFERENTIATION 



321 



cipal determining cause of the diversification of plant-forms. Let 

 us look a little further into this matter. Nature displays in the 

 island and in the continent the two influences of specialisation and 

 differentiation at work. We should look rather for specialised 

 genera in islands and for differentiated genera in continents. In 

 the first instance we have illustrated the Darwinian view, where a 

 species becomes so modified that it is given a generic rank; but 

 there is much evidence to show that this is a road which leads to 

 extinction, the specialised genus having, as a rule, a limited range 

 and a limited duration. In the case of the differentiated genus of 

 a continent we have a broad range and a promise of eternity. It is 

 of such that the plant- world was mainly formed in the past, is largely 

 composed in the present, and will be made in the future. Specialised 

 genera may figure conspicuously in localities; but their corpses 

 strew the path that nature has chiefly followed in the development 

 of the plant- world. 



The great trouble is that we give the same value to products of 

 very different origin and of very different standing ; the one transient 

 and limited in range, the other permanent and wide-ranging ; the one 

 representing nature's failures and the other her successes in stocking 

 the world with its plants. Whilst specialisation means extinction, 

 differentiation means a permanence of floral types that will hold the 

 world as long as there are conditions for plant-life. Take, for 

 instance, the monstrosities of the Tree Lobelias of Hawaii, of which 

 some half a dozen genera have been developed in that group. They 

 were born there and they will die there, and they have been unable to 

 extend their range. These specialised genera make no effort to 

 conquer the globe; yet we place them in a list of campanulaceous 

 genera side by side with such a world-ranging primitive genus as 

 Campanula. 



It would seem that monographers of orders may create their own 

 difficulties by not recognising this difference between specialised and 

 differentiated genera in a family. All their difficulties begin when 

 they try to bring them into line. The specialised genera should be 

 set apart and treated independently. 



Summary 



1 . The general nature and limitations of the differentiation theory 

 are briefly discussed, and the author's connection with it is described. 

 Stress is laid on his failure to recognise in his previous writings that 

 although it explains the diversity of plant-forms there is much in 

 distribution that it will not account for, distribution being also an 

 expression of the influence of the arrangement of the continents 

 during secular fluctuations of climate. The two subjects are accord- 

 ingly individualised and treated separately, under the heads of 

 Differentiation and Distribution, in this and the following chapter. 



2. The differentiation of a world-ranging generalised family type 

 is regarded as a response to the differentiation of originally uniform 

 conditions. The existing families are viewed as primitive and widely 

 distributed and derivative and relatively localised. It is considered 



Y 



