102 INVERTEBRATE PALAEONTOLOGY 



zones. Similarly Conifers and Lycopods, for the most 

 part, have abandoned competition with Angiosperms, 

 and prefer to brave the elements on mountain tops or in 

 high latitudes. With this evidence and by analogy 

 with individual life, it seems reasonable to suppose that 

 races of organisms demand some measure of shelter 

 during early and late stages of their development, and 

 join in the turmoil of robust activity only when racial 

 vitality is at its height. Choice of a sheltered environ- 

 ment by a group whose ancestors flourished amid 

 competition is equivalent to confession of failure ; 

 morphogenetic changes thus induced are rightly con- 

 sidered regressive, however they may harmonize with 

 the new surroundings. Development of adiposity or 

 arthritis is eminently appropriate for individuals whose 

 chief desire is for rest ; the diseases and the desire are 

 both symptomatic of old age and coming dissolution. 



Specialization can, therefore, be defined as modifica- 

 tion of structure conducive to success of an organism in 

 the most strenuous sectors of the battle-ground of life ; 

 regression as that connected with retirement to sheltered 

 surroundings. Both trends of evolution may be largely 

 adaptive, although both are probably influenced by 

 intrinsic qualities of racial rise and decay. The defini- 

 tion is neither complete nor exclusive, but it serves as 

 a useful datum whereby evolutional trends may be 

 provisionally correlated. 



The Ammonites have long been recognized as a 

 group affording an unusually complete and obvious 

 example of racial life-history. Although there is room 

 for difference of opinion as to the intrinsic or adaptive 

 qualities of the various modifications found in the group, 

 the general sequence of structural change is so regular 

 and so oft-repeated, that it must be ascribed to some 

 fundamental tendency. The straight, smooth, many- 



