INTRODUCTORY 7 



not species. But it is probable that this test would condemn a 

 great many wild species which are kept apart from others by the 

 clannishness of their members, not by intersterility. 



Artificial selection as practised by the breeder is not in any 

 way different in kind from Natural Selection. Unfortunately the 

 absolutely conclusive experiment, the development of a complex 

 animal from a one-celled organism, is altogether beyond man's 

 power. Natural Selection, therefore, remains a reasonable 

 hypothesis that can never be proved, but which commends itself 

 to a majority of competent judges. Among those who support 

 it, however, are some who while holding that it can account for 

 a great many of the phenomena of the animal and vegetable 

 kingdoms, yet deny that it can account for all. 



Before ending this section I will point out how curious has 

 been the evolution of opinion on the subject of evolution. 

 Before the publication of Darwin's Origin of Species, the theory 

 of evolution was much discussed but was generally rejected by 

 naturalists as it could not be shown what had brought it about. 

 Darwin's Natural Selection was welcomed as the working prin- 

 ciple which had made it possible. Darwinism, therefore, led 

 men to believe in evolution. A majority, probably a consider- 

 able majority of naturalists, are still Darwinians, but there are 

 now not a few who, while accepting evolution, are half inclined 

 to reject Darwinism, to scoff at the bridge by which the scientific 

 world made its way to evolution over a sea of difficulties. 



II 



THE EARLY STAGES OF EVOLUTION 



All living things are made up of minute cells and the simplest 



r 11 i l ar organ- 



that are known consist or one cell only. Among the unicellular ; sms 

 organisms I select for description one which, though not by 

 any means the least specialised, is yet low enough in the scale 

 to give some idea of the lowest forms of life, and at the 

 same time has the advantage of being easily obtainable. The 



