246 OUTLINES OF EVOLUTIONARY BIOLOGY 



We can only explain the occurrence of the same type of 

 skeleton and that a very complex type in all these different 

 kinds of organs of locomotion on the assumption that it has been 

 inherited from common ancestors. We cannot believe that one 

 and the same type of skeleton was necessarily the most suitable 

 for all these different cases, including ambulatory legs, wings and 

 paddles, and was therefore specially and independently created for 

 each. We must conclude rather that each organism receives a 

 certain kind of material by inheritance from its ancestors and 

 has to adapt it to its own requirements as best it may ; has, in 

 short, to cut its coat according to its cloth, and, whatever the 



* 



S.I. 



FIG. 102. A swimming Crab (Portunm depurator), showing jointed ambu- 

 latory appendages and also swimming appendages (s.L). (From a 

 photograph.) 



shape of the coat may ultimately develop into, the cloth will 

 retain, more or less evidently, traces of the original pattern. 



This conclusion is greatly strengthened when we turn to other 

 groups of animals and see how they have solved the same problems 

 with the aid of different materials. The vertebrates are not the 

 only animals which have organs of locomotion in the form of 

 jointed appendages. Many members of the great group 

 Arthropoda insects, crustaceans and spiders have ambulatory 

 limbs which externally bear considerable resemblance to those 

 of vertebrates. If we examine these limbs, however, we find that 

 they are constructed upon a wholly different plan. In the first 

 place the skeleton is external instead of internal, and is composed 



