ii.i DEVELOPMENT OF ANIMAL LIFE 133 



pods 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 considerable 

 — number of appendages, each of which has its special 

 function. In the vertebrates, activity is concentrated 

 in two pairs of members only, and these organs perform 

 functions which depend much less strictly on their form. 1 

 The independence becomes complete in man, whose hand 

 is capable of any kind of work. 



That, at least, is what we see. But behind what is 

 seen there is what may be surmised — two powers, im- 

 manent in life and originally intermingled, which were 

 bound to part company in course of growth. 



To define these powers, we must consider, in the evo- 

 lution both of the arthropods and the vertebrates, the 

 species which mark the culminating point of each. How 

 is this point to be determined? Here again, to aim at 

 geometrical precision will lead us astray. There is no 

 single simple sign by which we can recognize that one 

 species is more advanced than another on the same line 

 of evolution. There are manifold characters, that must 

 be compared and weighed in each particular case, in order 

 to ascertain to what extent they are essential or acci- 

 dental and how far they must be taken into account. 



It is unquestionable, for example, that success is the 

 most general criterion of superiority, the two terms being, 

 up to a certain point, synonymous. By success must be 

 understood, so far as the living being is concerned, an 

 aptitude to develop in the most diverse environments, 

 through the greatest possible variety of obstacles, so as to 

 cover the widest possible extent of ground. A species 



« See, on this subject, Shaler, The Individual, New York, 1900, pp. 

 118-125. 



