190 THE GENESIS OF SPECIES. [CHAP. 



horn, yet its furry coat is normal and ordinary. Again, the 

 Dugong and Manatee are dermally alike, yet extremely dif- 

 ferent as regards the structure and number of their teeth. 

 The porcupine also, in spite of its enormous armature of 

 quills, is furnished with as good a supply of teeth as are 

 the hairy members of the same family, but not with a bet- 

 ter one ; and in spite of the deficiency of teeth in the hair- 

 less dogs, no converse redundancy of teeth has, it is believed, 

 been remarked in Angora cats and rabbits. To say the 

 least, then, this 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 ac- 

 tion 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 no- 

 torious. Prof. Gegenbaur has lately endeavored 20 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 support- 

 ing 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 



20 " Ueber das Gliedmaassenskelet der Enaliosaurier, Jenaischen Zeit- 

 schrift," Bd. r. Heft 3, Taf. xiii. 



