MUTUAL AFFINITIES OF ORGANIC BEINGS 383 



been adapted for the same functions. A good instance is afforded 

 by the close resemblance of the jaws of the dog and Tasmanian 

 wolf or Thylacinus — animals which are widely sundered in the 

 natural system. But this resemblance is confined to general appear- 

 ance, as in the prominence of the canines, and in the cutting shape 

 of the molar teeth. For the teeth really differ much: thus the dog 

 has on each side of the upper jaw four pre-molars and only two 

 molars; while the Thylacinus 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 ad- 

 mitted 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 in- 

 sects possessing luminous organs — and of orchids and asclepiads 

 having pollen-masses with viscid disks, come under this same head 

 of analogical resemblances. But these cases are so wonderful that 

 they were introduced as difficulties or objections to our theory. In 

 all such cases some fundamental difference in the growth or de- 

 velopment of the parts, and generally in their matured structure, 

 can be detected. The end gained is the same, but the means, 

 though appearing superficially to be the same, are. essentially dif- 

 ferent. The principle formerly alluded to under the term of ana- 

 logical variation has probably in these cases often come into play; 

 that is, the members of the same class, although only distantly 

 allied, have inherited so much in common in their constitution, 

 that they are apt to vary under similar exciting causes in a similar 

 manner ; and this would obviously aid in the acquirement through 

 natural selection of parts or organs, strikingly like each other, 

 independently of their direct inheritance from a common pro- 

 genitor. 



As species belonging to distinct classes have often been adapted 

 by successive slight modifications to Hve under nearly similar cir- 

 cumstances — to inhabit, for instance, the three elements of land, 

 air, and water — we can perhaps understand how it is that a nu- 

 merical parallelism has sometimes been observed between the sub- 

 groups of distinct classes. A naturalist, struck with a parallelism of 

 this nature, by arbitrarily raising or sinking the value of the groups 

 in several classes (and all our experience shows that their valua- 



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