46 THE THEORY OF EVOLUTION 



before : they converge towards each other. Thus, 

 for instance, many mussels which by their construc- 

 tion belong to various species and genera, commence 

 at the same time or successively to alter their shells 

 in the same direction. Since now it is the shells alone 

 which are preserved, in those cases where the remainder 

 of the varied organism has left no trace we can no 

 longer know with what species or genera we have to 

 deal. 



Yet the similarity thus engendered is mostly only 

 superficial since it extends almost always only to the 

 shells, scales, and epidermal plates, as we see in the 

 Molluscs, Fishes, and Eeptiles ; or, since only some 

 of the organs for instance, those of locomotion are 

 similarly formed, the peculiarities of the whole 

 organization are never perfectly eliminated. It is, 

 therefore, by convergence that we explain how, within 

 quite different groups of mammalia, a most deceptive 

 similarity of the jaw construction is observed ; in this 

 case this is undoubtedly caused by habituation to the 

 same kind of nutrition. 1 



By convergence is explained also the entire ' fish 

 resemblance ' of the Whales, which, when habituating 

 themselves to life in the water, were guided, so to 

 speak, by the water inhabitants par excellence, the 

 Fishes that is, they changed their forelimbs (arms) into 



1 If only separated plates (shields) or separate teeth are found then we 

 cannot tell to which animal group they should be assigned. Hence recently 

 a polygonal epidermal plate was ascribed by Filhol to an extinct armadillo 

 (Mammalia), but later an almost exact replica was found, but this time on 

 the head of a reptile (Placosaurus). Umbildung der TierwelL p. 208. 



