72 MODERN IDEAS OF EVOLUTION 



carbonic dioxide and ammonia of the atmosphere, 

 with the aid of solar energy? This problem has 

 been practically solved perhaps many times and 

 under different conditions by creative power, but no 

 evolutionist has yet explained it, and the careful 

 experiments of Pasteur and Tyndall have given only 

 negative results. 



These plants are, however, capable of certain 

 variations. The Protococcus may differ somewhat in 

 colour, or in proportion of parts in different circum 

 stances, and it is not impossible that some of the 

 forms which have been described as distinct species 

 arc really merely varieties of this kind. This might 

 of course enable a botanist to speak of different 

 species of Protococcus as having originated by descent 

 with modification ; but if the different forms could be 

 shown to be merely the result of changed conditions, 

 and to be capable of returning in suitable circum 

 stances to the normal properties of the plant, they 

 could not be regarded as true species. He might, 

 however, farther argue that under circumstances 

 of isolation, and where external influences permitted 

 only one form to exist, this might become fixed and 

 continuously reproductive as a distinct species ; but 

 in that case the burden of proof would rest with him, 

 and such proof has not yet been obtained. Until it 

 has, the independent origin of such forms remains 

 quite as possible. If a one-celled alga could be pro 

 duced de novo on the surface of Greenland snow, why 



