464 ORIGIN OF SPECIES 



is confined to general appearance, 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; 

 whilst 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 

 \videly 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 with viscid discs, come 

 under this same head of analogical resemblances. But these 

 cases are so wonderful that they were introduced as diffi- 

 culties or objections to our theor}'. In all such cases some 

 fundamental difference in the growth or development of the 

 parts, and generally in their matured structure, can be de- 

 tected. The end gained is the same, but the means, though 

 appearing superficially to be the same, are essentially differ- 

 ent. The principle formerly alluded to under the term of 

 analogical 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 obvi- 

 ously aid in the acquirement through natural selection of 

 parts or organs, strikingly like each other, independently of 

 their direct inheritance from a common progenitor. 



As species belonging to distinct classes have often been 

 adapted by successive slight modifications to live under 

 nearly similar circumstances, — to inhabit, for instance, the 

 three elements of land, air, and water, — we can perhaps un- 

 derstand how it is that a numerical parallelism has sometimes 

 been observed between the sub-groups of distinct classes. A 



