ii6 INVERTEBRATE PALAEONTOLOGY 



corresponding with infancy, childhood and adolescence. 

 The episodes of these stages mark the progress of an 

 organism towards the degree of development attained 

 by the group of which it is a member. In common 

 phraseology, a newborn infant may be said to "start 

 late," so that it needs must " hurry " to catch up the main 

 group of its fellows. But speedy as early progress may 

 be, it leads along the same course that was taken 

 during phyletic anagenesis of the stock. The dominant 

 characteristics of youth can be ascribed to exuberant 

 vitality. The child has to find its place in race-life, and 

 to fit itself for the activities of maturity. Structurally 

 it not only grows, but develops; psychologically it 

 learns, chiefly by experiment, the only sound method. 

 Habits, the subconscious sequels to experience, are 

 acquired ; some make for success, others may be harmful. 

 If maturity is to bring full fruition, good habits must be 

 confirmed and extended, bad ones dropped. The most 

 enduring impressions are those gained in infancy ; they 

 are concerned with fundamentals, and their nature may 

 influence the course of all later development of character. 

 As childhood passes to adolescence, details are added 

 to the early outline; the rate of their introduction 

 becomes slower until maturity is reached. 



It is clear that this sketch of early individual life 

 could be applied, almost verbatim, to phyletic anagenesis, 

 replacing the words infancy, childhood and adolescence 

 by their phyletic equivalents neanic, nepionic and 

 epacmaic stages. The rate and quality of differentia- 

 tion in a stock are greatest in the neanic stage, when 

 most ordinal (and even family) series are evolved. It is, 

 moreover, particularly characteristic of this stage that 

 great numbers of short-lived, often very aberrant, 

 branches arise the "unsuccessful experiments" of 

 infant life. After neanic acceleration follows relatively 



