198 THE GENESIS OF SPECIES. [CHAP. 



law of correlation presents numerous and remarkable 

 exceptions. 



To return, however, to the subject of homological rela- 

 tions : it is surely inconceivable that indefinite variation 

 with survival of the fittest can ever have built up these 

 serial, bilateral, and vertical homologies, without the action 

 of some special innate power or tendency so to build up, 

 possessed by the organism itself in each case. By " special 

 tendency " is meant one the laws and conditions of which 

 are as yet unknown, but which is analogous to the innate 

 power and tendency possessed by crystals similarly to 

 build up certain peculiar and very definite forms. 



First, with regard to comparative anatomy. The cor- 

 respondence between the thoracic and pelvic limbs is 

 notorious. Professor Gegenbaur has lately endeavoured x 

 to explain this resemblance by the derivation of each limb 

 from a primitive form of fin. This fin is supposed to have 

 had a marginal external (radial) series of cartilages, each 

 of which supported a series of secondary cartilages, starting 

 from the inner (ulnar) side of the distal part of the sup- 

 porting marginal piece. The root marginal piece would 

 become the humerus or femur, as the case might be ; the 

 second marginal piece, with the piece attached to the 

 inner side of the distal end of the root marginal piece, 

 would together form either the radius and ulna or the tibia 

 and fibula, and so on. 



Now there may be little doubt (from a priori considera- 

 tions) as to the special differentiation of the limb-bones of 

 the higher Vertebrates having been evolved from anterior 

 conditions existing in some fish-like form or other. But 



i " Ueber das Gliedmaassenskelet der Enaliosaurier," Jenaischen Zeit- 

 schrift, Bd. v. Heft 3, Taf. xiii. 



