180 



THE GENESIS OF SPECIES. 



[CHAP. 



ity of these it is very difficult, if not impossible, to see 

 how external conditions can have produced, or even 

 tended to have produced them. For example, we may take 

 the migration of one eye of the sole to the other side 

 of its head. What is there here either in the darkness, or 

 the friction, or in any other conceivable external cause, to 



PLETTBONECTID^, WITH THE PECULIARLY-PLACED EYE IN DIFFERENT POSITIONS. 



have produced the first beginning of such an unprecedented 

 displacement of the eye ? Mr. Spencer has beautifully 

 illustrated that correlation which all must admit to exist 

 between the forms of organisms and their surrounding exter- 

 nal conditions, but by no means proved that the latter are 

 the cause of the former. 



Some internal conditions (or in ordinary language some 

 internal power and force) must be conceded to living organ- 

 isms, otherwise incident forces must act upon them and 

 upon non-living aggregations of matter in the same way, and 

 with similar effects. 



If the mere presence of these incident forces produces 

 so ready a response in animals and plants, it must be 

 that there are, in their case, conditions disposing and 

 enabling them so to respond, according to the old maxim, 



