6 INTRODUCTION. 



attributes which have been preserved by transmission through 

 long series of apparently very different and yet nearly allied 

 species, and those characters of adaptation which here and there 

 arise, as it would seem arbitrarily, and beyond a doubt quite 

 independently of the affinities of animals. The absolute 

 necessity of clearly separating these two groups of characters 

 will be made plain by the following illustration of one special 

 case in point. 



Everyone knows that the lungs of all the higher vertebrate 

 animals are indispensable to their existence ; no mammal, bird, 

 or even reptile, could live long without this breathing apparatus. 

 Similarly constructed and indispensable organs for breathing air 

 occur in many Mollusca and in a few Crustaceans. Now, if a 

 zoologist endeavoured to prove that all animals which have 

 organs adapted to respire air must therefore be closely allied, 

 it would hardly be worth while to point out that his attempt 

 must be hopelessly futile. It suffices, with regard to the 

 example I have adduced, to point to the fact that the lungs of 

 the Vertebrata are connected with the intestinal canal and 

 developed from it; while those of Mollusca and Crustaceans 

 (fig. 2) are nothing more than cavities in the side, which have 

 originated from a lateral invagination of the outer skin. But 

 an organ which, like the lungs of mammals and birds, takes its 

 rise from the intestinal canal, can never have originated in a 

 modification of the outer skin, or epidermis. This proves that 

 the lungs of different groups of animals must have originated 

 independently of each other, and that we are justified in regard- 

 ing them as, in some degree, characters of adaptation. 



Another instance. It is now universally admitted that the 

 fore limbs of mammals, the wings of birds, and the pectoral fins 

 of fishes are, morphologically, merely modifications of the same 

 organs, namely, the fore or pectoral limbs. They terminate jn 

 man in a hand, in the apes in a hand serving also as a foot, 

 in the horse in a foot only, in birds in an organ of flight, in 

 fish in an organ of swimming. In all these cases the function 

 of the limbs is different, although they .are morphologically 

 identical. Furthermore, all fins are not morphologically iden- 

 tical, and if we were to attempt to regard the pectoral fins of 



