204 EVOLUTION AND ANIMAL LIFE 



rabbits, owls, ptarmigans, and other birds and mammals, this 

 color aiding alike in defense or attack as against the back- 

 ground of snow. Similar convergence of characters is seen in 

 the gray hues of almost all desert animals, in the thorny stems 

 and scant thick foliage of almost all desert plants. In swift 

 streams, fishes of various types (sculpins, darters, gobies, cat- 

 fish, and minnows) protect themselves from the current by the 

 reduction of the air bladder, by the instinct to lie flat on the 

 bottom, and the instinct to make short quick darts from place to 

 place in the swift waters. To this end also, certain fins are in 

 each case especially increased in size and force. 



Convergence of characters is shown in the black colors, soft 

 bodies, and luminous spots, characteristic of different groups of 

 deep-sea fishes. It also appears in the development of eellike 

 forms in groups of fishes which have no affinity with eels, and 

 of snakelike forms among .lizards and salamanders, which have 

 no real affinity with snakes. Like conditions of life bring about 

 like structures. We may instance the occurrence of blind fishes 

 of various groups in the different cave areas, these species being 

 derived in all cases from fishes of neighboring regions having 

 well-developed eyes. Thus the blind cave fish of Missouri (Trog- 

 lichthys roscc), and those of Indiana and Kentucky (Amblyopsis 

 spelccus, Typhlichthys subterraneus) , are separately derived from 

 the once widespread type of the Dismal Swamp fish (Cholo- 

 gaster cormdus). The blind fishes of Cuba (Stygicola, Lucifiiga} 

 are derived from ancestors of a marine cusk (Brotula) now found 

 in the Cuban seas. The blind catfish of Pennsylvania (Gronias 

 nigrilabris) is modified from an existing species (Ameiurus 

 pnllus) found in the same region. The blind salamanders of 

 Austria and Texas are derived from former inhabitants of the 

 same regions which possessed well-developed eyes. 



Parallelisms of this sort are found in every group of animals 

 and plants. It is generally easy to distinguish analogous 

 variations or results of convergence of characters, by the study 

 of comparative anatomy. Resemblances induced by like selec- 

 tion or by like conditions are usually superficial and do not 

 affect those structures which do not come into direct contact 

 with external conditions. But sometimes even deep-seated 

 characters have been reached and affected by environmental 

 influences. In this case a finer test is found in the study of 

 embryonic development. In general, creatures actually closely 



