110 ORGANIC EVOLUTION 



be regarded as immeasurably superior, as well as from 

 the point of view of morphology. To this generalisa- 

 tion the social insects, such as the bee and the ant, 

 offer a limited but extremely striking exception. 

 Nevertheless, even whilst recognising that their 

 claims are not lightly to be set aside, we may pro- 

 ceed to confine ourselves, in answering the question 

 whether physical evolution has reached its goal, 

 exclusively to the vertebrates. 



Assuming, then, for the nonce at any rate, that 

 we may justly answer this question with sole refer- 

 ence to the vertebrate family, we may attempt to 

 express, in very broad outline, the general tendency 

 of physical evolution in this family. And we find 

 it possible to frame an exceedingly simple expression 

 thereof. Characteristic of the vertebrates is the 

 possession of two pairs of limbs. Now as we survey 

 the whole family, from the fish to man, we find 

 that there is a tendency to specialisation — i.e. to 

 evolution in regard to the structure and function 

 of these limbs. The posterior pair alone tend to 

 discharge the function of locomotion upon the 

 ground, which was previously discharged by an- 

 terior and posterior limbs alike. The pair of limbs 

 which are nearest to the supremely important brain 

 and to the very important mouth — i.e. the fore-limbs 

 — tend to assume more complicated functions. If 

 we briefly trace the history of the vertebrate groups 

 this becomes apparent. From the fish there is 

 evolved the amphibian, which spends only its 

 earlier stages in water, and is an air-breather when 

 adult. 1 From it proceeds the reptile, which in its 



1 Speaking very broadly, we may say that the tadpole is a fish, 

 the (adult) frog a reptila 



