36 THE GROWTH OF THE BRAIN. 



same time birds, which in many instances are credited 

 with more than a hundred years, have not been shown 

 to undergo this long-continued growth. In the case of 

 birds, therefore, neither continuous growth nor large size 

 can be called in to explain long life, so that in reality 

 the inherent conditions which determine longevity are 

 still unrecognised. 



Nevertheless vertebrates may be divided into two 

 principal groups, according to the manner in which 

 they grow. There are those in which the size of the in- 

 dividual is fixed, in the sense that some time long before 

 the natural close of life growth ceases; and those in which 

 growth changes are almost coextensive with life. To 

 the former class man belongs, together with the other 

 mammals and the birds, and to the latter some fish, 

 amphibia, and reptiles. 



It has been already stated that there are two ways in 

 which an organism may increase in size : either by the 

 increase of the number of cells composing it, or the 

 number remaining the same, by an increase in their mass. 

 As a matter of fact, the two sets of changes always go 

 hand in hand, but the share they take in the final result is 

 not the same in different animals or different tissues, nor 

 is it the same at the various periods in the life history 

 of the same animal. The topic is worthy of some 

 elaboration. 



The different classes of vertebrates are similar in the 

 fact that during the early life of the embryo, before the 

 organs are formed, the chief factor giving rise to the 

 increase in size is the production of new cells. As this 

 production becomes less rapid, those cells which have 

 been already formed undergo enlargement, together 

 with such changes in structure as fit them for their 

 special duties. The first process is thus gradually 

 replaced by the second. In the human nervous system 



