132 PRIMITIVE FORMS OF LIFE 



another and carnivorous group of Reptiles of the same Period, 

 called Theropods. Many of these animals had bones which were 

 hollow or penetrated by the diverticula of the air-spaces 

 which are now peculiar to Birds, so that the respiratory 

 apparatus itself, so long considered as destined to obtain for the 

 bird the energy required for flight, is seen to have had no 

 original connexion with this special method of locomotion. 

 None of the characters that class an organism as a bird were 

 assembled for its present peculiar mode of life ; the feathers 

 arose from the multiplication of the cells of the epidermis 

 and their faculty of producing abundantly a horny substance ; 

 the conformation of their hind legs results from the advantage 

 the beast found in standing erect upon them and in hopping, 

 and the augmentation of activity it developed for this purpose 

 reacted upon the respiratory apparatus. Thus far, it might seem 

 that the animal was a kind of patchwork ; but once these 

 characters were all united, the Reptile, having become a 

 hopping creature making use of its feathered front limbs 

 as parachutes, was able to support itself in the air, as, by 

 different methods, the Insects and the Pterodactyls had 

 succeeded in doing, and as the Flying Fishes and the Bats 

 succeeded in doing later. Thus, from the fortuitous reunion 

 of a set of characters and organic arrangements, developed 

 without any end in view, or, at least, with an end other than 

 that of flight, the Bird subsequently perfected itself through 

 the exercise of these characters. We must, then, guard against 

 the belief that a single category of causal factors, a single 

 process, or a single method has sufficed to create the diverse 

 forms of living creatures, and that any one theory can account 

 for their evolution. All these living forms that surround us 

 are the result of a gargantuan conflict of forces and substances, 

 greater even than what we call the struggle for existence — 

 a conflict compared with which the history of peoples and races, 

 complex as it appears to us, is but a picture seen through a 

 diminishing glass. Nor must we forget that even in the case of 

 what are called pre-adaptations, the animal can only profit 

 by the new characters it has acquired by using its muscles and 

 its nervous system in a new way. It depends on itself whether 

 it makes the best use of these various features of its organization. 

 Adaptation to environment, initiated before its own volition 

 comes into play, is finally achieved only by this volition, 



