132 CREATIVE EVOLUTION [chap. 



place and the moment of encounter. We see a progress 

 of the same kind in the evolution of human armaments. 

 The first impulse is to seek shelter; the second, which is 

 the better, is to become as supple as possible for flight and 

 above all for attack — attack being the most effective 

 means of defense. So the heavy hoplite was supplanted 

 by the legionary; the knight, clad in armor, had to give 

 place to the light free-moving infantryman; and in a 

 general way, in the evolution of life, just as in the evo- 

 lution of human societies and of individual destinies, the 

 greatest successes have been for those who have accepted 

 the heaviest risks. 



Evidently, then, it was to the animal's interest to 

 make itself more mobile. As we said when speaking 

 of adaptation in general, any transformation of a species 

 can be explained by its own particular interest. This 

 will give the immediate cause of the variation, but often 

 only the most superficial cause. The profound cause is 

 the impulse which thrust life into the world, which made 

 it divide into vegetables and animals, which shunted the 

 animal on to suppleness of form, and which, at a certain 

 moment, in the animal kingdom threatened with torpor, 

 secured that, on some points at least, it should rouse itself 

 up and move forward. 



On the two paths along which the vertebrates and 

 arthropods have separately evolved, development (apart 

 from retrogressions connected with parasitism or any 

 other cause) has consisted above all in the progress of 

 the sensori-motor nervous system. Mobility and sup- 

 pleness were sought for, and also — through many experi- 

 mental attempts, and not without a tendency to excess 

 of substance and brute force at the start — variety of move- 

 ments. But this quest itself took place in divergent 

 directions. A glance at the nervous system of the arthro- 



