58 ORGANIC EVOLUTION' 



us consider, in especial, his fore-legs. It is the 

 characteristic of vertebrated animals — all of whom, 

 on the evolutionary theory, have a common origin 

 — to possess two pairs of limbs. The anatomical 

 parallelism between such limbs, wherever met, is 

 technically known as homology} Thus the fore-limb 

 of a horse is the homologue of the fore-limb of man 

 or the frog or the bat. Now there is a well-marked 

 type to which the vertebrate fore-limb — the case of 

 the hind-limb is quite parallel — in general conforms. 

 In the upper segment of the limb there is one bone, 

 the humerus; in the next segment there are two 

 bones, the radius and ulna, to which is attached 

 the wrist, usually consisting approximately of two 

 rows of four small bones each ; and to the wrist is 

 attached the hand, which is a five-fold structure. 

 The vertebrate hand (and foot) is typically five- 

 lingered, or, to use the exact Greek equivalent, 

 pentadigitate. The hand with which you are 

 holding this book, and the hand of a frog, are the 

 first instances that occur to me. 



But there are exceptions, you will say ; to which 

 I retort that, for once in a way, we can attach a real 

 meaning to the proverb, usually quoted without any 

 meaning, that the " exception proves the rule." The 

 exceptions to this rule really do prove it ; which is 

 indeed to say that they conform to it, appearances 

 notwithstanding. Let us consider the case of the 

 birds (I have not forgotten the horse, though I 



1 The term analogy is applied to organs which serve the same 

 function in different species, though their place in the structure of 

 the animal may be dissimilar. Thus the wing of a bird and the arm 

 of a man are homologous, but not analogous. The skeleton of a 

 man and that of a lobster are analogous, but not homologous. 



