ii DEVELOPMENT OF ANIMAL LIFE 139 



the heavy hoplite was supplanted by the legionary ; the 

 knight, clad in armour, had to give place to the light 

 free-moving infantryman ; and in a general way, in the 

 evolution of life, just as in the evolution of human 

 societies and of individual destinies, the greatest 

 successes have been for those that 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 suppleness were sought for, and also 

 through many experimental attempts, and not with 

 out a tendency to excess of substance and brute 

 force at the start variety of movements. But this 

 quest itself took place in divergent directions. A 

 glance at the nervous system of the arthropods and that 

 of the vertebrates shows us the difference. In the 

 arthropods, the body is formed of a series more or 

 less long of rings set together ; motor activity is thus 

 distributed amongst a varying sometimes a con- 



