114 DARWINISM chap. 



amounts, there seems no reason to believe that the necessity 

 for two or more coincident variations would prevent the 

 required change from taking place. 



The Continued Existence of Low Forms of Life. 



Since species are continually undergoing modifications 

 giving them some superiority over other species or enabling 

 them to occupy fresh places in nature, it may be asked — Why 

 do any low forms continue to exist ? Why have they not long 

 since been imj^roved and developed into higher forms ? The 

 answer, probably, is, that these low forms occupy places in 

 nature which cannot be filled by higher forms, and that they 

 have few or no competitors ; they therefore continue to 

 exist. Thus, earthworms are adapted to their mode of life 

 better than they would be if more highly organised. So, in 

 the ocean, the minute foraminifera and infusoria, and the 

 larger sponges and corals, occupy places which more highly 

 developed creatures could not fill. They form, as it were, the 

 base of the great structure of animal life, on which the next 

 higher forms rest; and though in the coiu-se of ages they 

 may undergo some changes, and diversification of form 

 and structure, in accordance with changed conditions, their 

 essential nature has probably remained the same from the 

 very dawn of life on the earth. The low aquatic diatomaceae 

 and confervas, together with the lowest fungi and lichens, 

 occupy a similar position in the vegetable kingdom, filling 

 places in nature which would be left vacant if only highly 

 organised plants existed. There is, therefore, no motive 

 power to destroy or seriously to modify them ; and they have 

 thus probably persisted, under slightly varying forms, through 

 all geological time. 



Extinction of Lower Types among the Higher Animals. 



So soon, however, as we approach the higher and more 

 fully developed groups, we see indications of the often re- 

 peated extinction of lower by higher forms. This is shown 

 by the great gaps that separate the mammalia, birds, reptiles, 

 and fishes from each other ; while the lowest forms of each are 

 always few in number and confined to limited areas. Such 



