276 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



on and say that in vertebrates the limbs are reduced to 

 two pairs, and these are therefore quadrupeds. This is as 

 far as reduction can go for highest locomotive efficiency 

 on land. In birds locomotive efficiency on land is sacri- 

 ficed for flight; and in man it is sacrificed for efficiency 

 of another and still higher kind, and the reduction goes 

 on to one pair, the other pair common to vertebrates 

 being set free for higher uses as wings and hands. 



Law of Differentiation. — We have already seen 

 (page 23) that cells, commencing in the lowest animals 

 and in the earliest embryonic condition, all alike, and per- 

 forming each all the functions of life but very imper- 

 fectly, as we rise in the scale become differentiated in 

 form and specialized in function, each doing its own 

 work and doing it much better. So now we find the 

 same law in the segments. In the earliest geological 

 times, and in the lowest animals now, we find all the 

 segments alike and performing similar functions, but 

 imperfectly. But as we go up the scale the segments 

 and their appendages are more and more differentiated 

 in form and function, and by this division of labor the 

 functions are better performed. The same law may be 

 carried still a step further. Regarding a whole depart- 

 ment, such as the vertebrata or articulata, as composed 

 of a repetition of organisms made on the same plan ; 

 then in early geological times there was doubtless far 

 more similarity than now. In the process of evolution 

 these somewhat similar organisms were differentiated, 

 each kind for its own place in the economy of Nature, 

 and specially fitted for that place through the survival 

 of the fittest. This law of differentiation, therefore, 

 is the most fundamental and all-pervasive law of evo- 

 lution. 



Nervous System. — We have confined ourselves thus 

 far to the skeletal system. Next to this, as illustrat- 



