HIGHWAY OF ANIMAL EVOLUTION 237 



heart, locomotor, respiratory, sexual, excretory, and di- 

 gestive organs became greatly altered in relative size 

 and relative position, and all of them gradually with- 

 drew from their primitive centres of activity in the 

 head, leaving that important region almost exclusively 

 to the receptive functions and to the main centres of 

 nerve control. Some of them, such as the heart, lungs, 

 and locomotor organs, centralized their activities at 

 such points as the services to be performed demanded; 

 the kidneys, reproductive and other organs, where 

 functional location is not so imperative, taking their 

 positions at otherwise unoccupied regions of the body. 



The general effect of all these organic readjust- 

 ments was to transform the growing series of more or 

 less independent metameres into a more unified body 

 wherein each important function was performed by 

 that one set, or pair of organs which was so situated 

 as to give the maximum bodily service. It was like 

 changing a long row of tenement houses into one resi- 

 dence, with centralized systems for the receipt, prep- 

 aration, and distribution of common necessities. 



The ultimate location of each centralized function 

 is determined by various factors; chiefly by priority 

 of origin, which in turn is determined by the neces- 

 sity of prior activity in primitive or in embryonic life; 

 by the imperative demands for a special location, in 

 order rightly to perform its work; and by the univer- 

 sal organic law that the main lines of growth coincide 

 with the most available channels of conveyance. 



I have described these changes more specifically in 

 "The Evolution of the Vertebrates and Their Kin." A 

 careful consideration will show us that in man no fur- 

 ther morphological or functional changes of this kind 



