ii.] DEVELOPMENT OF ANIMAL LIFE 129 



only at its own convenience. It goes for that which 

 demands the least labor. Absorbed in the form it is 

 about to take, it falls into a partial sleep, in which it 

 ignores almost all the rest of life; it fashions itself so 

 as to take the greatest possible advantage of its immediate 

 environment with the least possible trouble. Accord- 

 ingly, the act by which life goes forward to the creation 

 of a new form, and the act by which this form is shaped, 

 are two different and often antagonistic movements. 

 The first is continuous with the second, but cannot con- 

 tinue in it without being drawn aside from its direction, 

 as would happen to a man leaping, if, in order to clear 

 the obstacle, he had to turn his eyes from it and look at 

 himself all the while. 



Living forms are, by their very definition, forms that 

 are able to live. In whatever way the adaptation of the 

 organism to its circumstances is explained, it has necessa- 

 rily been sufficient, since the species has subsisted. In 

 this sense, each of the successive species that paleon- 

 tology and zoology describes was a success carried off by 

 life. But we get a very different impression when we 

 refer each species to the movement that has left it behind 

 on its way, instead of to the conditions into which it has 

 been set. Often this movement has turned aside; very 

 often, too, it has stopped short; what was to have been 

 a thoroughfare has become a terminus. From this new 

 point of view, failure seems the rule, success exceptional 

 and always imperfect. We shall see that, of the four 

 main directions along which animal life bent its course, 

 two have led to blind alleys, and, in the other two, the 

 effort has generally been out of proportion to the result. 



Documents are lacking to reconstruct this history in 

 detail, but we can make out its main lines. We have 

 already said that animals and vegetables must have 



