72 EVOLUTION OF TO-DAY, 



ing, of course, its fundamental features, but chan- 

 ging its shape to suit the change in function. On 

 the other hand, the descendants remaining terres- 

 trial, the limb became a grasping organ, likewise 

 retaining its fundamental structure. Thus the the- 

 ory of descent with modification explains both the 

 similarities and the differences between homologous 

 organs, the former being due to inheritance and the 

 latter to variation. And as far as instances of this 

 kind are concerned the explanation is certainly- 

 sufficient. 



But there is another very different class of homol- 

 ogies which cannot be thus explained. What can 

 be said of the homology which exists between the 

 arm and leg of a man, between the wing and leg of 

 a bird, between the members of the long series of 

 legs of the lobster in short, between all sorts 

 of serial organs ? Here is a\ homology as exact 

 as that in the case considered above. Every bone 

 in the arm has its homologue in the leg ; every ap- 

 pendage of the lobster is strictly comparable, joint 

 by joint, with all the others. Indeed, the exactness 

 of this serial homology is frequently more complete 

 than that of the general homologies above re- 

 ferred to, and it is very evident that this type of 

 homology requires an explanation just as truly as 

 did the first. But the explanation given to the first 

 case will not apply here. Let us see what would 

 be the result. Using the above explanation, we 

 should say that the arm and leg owe their likeness 

 to the fact that they have both descended from 

 a common ancestor, which possessed the funda- 



