III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 75 



dently developed. It is so because the number of possible 

 variations is indefinitely great, and it is therefore an inde- 

 finitely great number to one against a similar series of 

 variations occurring and being similarly preserved in 

 any two independent instances. 



The difficulty here asserted applies exclusively, however, 

 to pure Darwinism, which makes use only of indirect 

 modifications through the survival of the fittest. 



A CENTIPEDE. 



Other theories (for example, that of Mr. Herbert Spencer) 

 admit the direct action of conditions upon animals and 

 plants in ways not yet fully understood there being con- 

 ceived to be at the same time a certain peculiar but limited 

 power of response and adaptation in each animal and plant 

 so acted on. Such theories have not to contend against 

 the difficulty proposed, and it is here urged that even very 

 complex extremely similar structures have again and again 

 been developed quite independently one of the other, and 

 this because the process has taken place not by merely 

 haphazard, indefinite variations in all directions, but by the 



