220 CLASSIFICATION. [Chap. XIV. 



for swimming tlirough the M'ater; but between tlie 

 several members of the whale family, the shape of the 

 body and the fin-like limbs offer characters exhibiting 

 true affinity; for as these parts are so nearly similar 

 throughout the u*hole family, we cannot doubt that they 

 have been inherited from a common ancestor. So it is 

 with fishes. 



Numerous cases could be given of striking resemblances 

 in quite distinct beings between single parts or organs, 

 which have been adapted for the same functions. A 

 good instance is afforded by the close resemblance of the 

 jaws of the dog and Tasmanian woK or Tliylacinus, — 

 animals which are widely sundered in the natural sys- 

 tem. But this resemblance is confined to general appear- 

 ance, as in the prominence of tlie canines, and in the 

 cutting shape of tlie molar teeth. For tlie teeth really 

 differ much : thus the dog has on each side of the upper 

 jaw four pre-molars and only two molars; whilst the 

 Tliylacinus has three pre-molars and four molars. The 

 molars also differ much in the two animals in relative 

 size and structure. The .adult dentition is preceded by 

 a widely different milk dentition. Any one may of 

 course deny that the teeth in either case have been 

 adapted for tearing flesh, through the natural selection 

 of successive variations ; but if this be admitted in the 

 one case, it is unintelligible to me that it should be 

 denied in the other. I am glad to find that so high an 

 authority as Professor Flower has come to this same 

 conclusion. 



The extraordinary cases given in a former chapter, of 

 widely different fishes possessing electric organs, — of 

 widely different insects possessing luminous organs, — 

 and of orchids and asclepiads having pollen-masses 



