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 peculiar 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 
Fic. 304.—White Water Crowfoot (Ranunculus aquatilis, var. capillaceus, 
Crowfoot Family, Ranunculacee). Plant, about 3. Flower. Fruit. 
(Britton and Brown.)—Perennial (?) herb about 30 cm. long; leaves 
submerged; flowers white; fruit dry. Native home, North America 
and Eurasia. 
fitting the individuals to live 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 
