EVOLUTION IN GENERAL 



467 



Many types of structure have become extinct, because 

 changing conditions no longer afforded a suitable environ- 

 ment, or, perhaps because no mutations of the old form could 

 adapt it to new circumstances of pecuHar difficulty. Relics 

 of types which the world has thus outgrown have occasionally 

 come down to us as fossils caught in the deposits which be- 

 came rock in ages past. 



Sometimes a group, or perhaps part of its members, may 

 have escaped extinction through the appearance of mutations 



Fig. 304. — White Water Crowfoot (Raiiuiiculufi aquatilis, var. capillaceus. 

 Crowfoot Family, Ranunculacece) . Plant, about J. Flower. Fruit. 

 (Britton and Brown.) — Perennial (?) herb about 30 cm. long; leaves 

 submerged; flowers wliite; fruit dry. Native home. North America 

 and Eurasia. 



fitting the individuals to five under less exacting conditions 

 which therefore would permit simpler structure. Thus a 

 buttercup able to live in water without being drowned could 

 dispense with much of its root system and stiffening frame- 

 work and so come to resemble, in the adaptation of its vege- 

 tative organs to an aquatic life, a lower form of plant none of 

 whose ancestors had been terrestrial. The white water 

 crowfoot (Fig. 304) of our ponds and streams is a buttercup 

 which we have every reason to believe has thus descended 

 from a land species. In so far as a type of organism or organ 



