ISOLATION 363 



That these two species are descendants of a common stock 

 there can be no question. And we may safely assume that the 

 Indian parti-coloured species is the more primitive of the two. 

 The African bird, then, isolated by migration, has not only 

 intensified the peculiarities of the beak, but has further de- 

 veloped the striking changes of plumage just referred to. The 

 African species, it is further to be noted, occurs throughout the 

 whole of the continent and Madagascar, yet examples from 

 any part of this vast range exhibit the same characters. It is 

 true an attempt has been made to show that examples from 

 Madagascar can be distinguished from specimens from the 

 continent of Africa. Dr. Bowdler Sharpe insists that such dis- 

 tinctions cannot be admitted. The Indian species, which 

 ranges into Ceylon and Cochin-China, exhibits a similar con- 

 stancy, retaining its characteristic parti-coloured plumage. 



The evolution of the African species can scarcely be put 

 down to the influence of natural selection. But it may well be 

 attributed to the indirect effect of discriminate isolation, whereby 

 certain individuals, having the latent characters of the African 

 species, were enabled to develop these innate peculiarities. 



If this be admitted, we must endeavour to find some other 

 interpretation of the peculiarities of the African species, and their 

 persistence practically unchanged, throughout its whole enor- 

 mous range. And what applies in this instance will apply also 

 to a vast number of similar cases. 



It is suggested then, that the peculiarities which distin- 

 guish the African bird are due to the removal — consequent on 

 migration from the ancestral home — of certain restraining in- 

 fluences checking the further development of an inherent tend- 

 ency to blackness of plumage, and an intensification of the 

 peculiar form of the beak : a development which but for some 

 subtle check might have become excessively developed and so 

 brought about extinction. 



This assumed inherent tendency of organs to pursue a 

 certain definite course of evolution, once started, is known as 

 " determinate evolution," but this factor is not regarded with 

 general favour among evolutionists. Nevertheless, there seems 

 very good reason for assuming that organs, like individuals, vary 

 in their potentialities of growth ; and that once started in a 

 gi%'en direction, this growth will continue until, and unless, 



