u8 INVERTEBRATE PALAEONTOLOGY 



perhaps some Primates) suffer loss of dentition. Many 

 show phyletic reversion, none the less analogous with 

 second-childhood because incomplete. The Spiriferacea 

 have already been cited in illustration of the outburst 

 of specialization (of generic and even family quality) 

 that precedes extinction ; many other groups might 

 have been used for the purpose. 



It is possible, therefore, to summarize the matter 

 included in this chapter by the suggestion that individual 

 life does present, in epitome, the salient characters of 

 phylogeny. The intensity of development varies in 

 individuals no less than in groups. There are, roughly, 

 two extreme types of individual life ; that often recom- 

 mended as "short and merry," and that "humdrum" 

 existence that has usually been the lot of an "oldest 

 inhabitant." Much may be accomplished during the 

 brief course of the former type ; little achievement, long 

 drawn out, characterizes the latter. So in phylogeny 

 the stocks that rise rapidly to prominent acmes fall 

 precipitately; they have short, but effective, careers. 

 Those groups that climb slowly and without distinction 

 to inglorious maturity linger through an uneventful and 

 protracted period of senescence. Accident may shorten, 

 or shelter prolong, the lives of individuals; evidence 

 is not wanting that comparable circumstances have 

 influenced phyletic evolution. 



Life, whether individual or racial, seems normally to 

 follow a parabolic course. An organism rises from birth 

 to maturity, and then, with like speed, sinks back to the 

 unknown from whence it sprang. A phyletic stock 

 pursues a similar course, anagenetic and catagenetic, 

 from inception to extinction. The analogy is too perfect 

 to be the outcome of mere coincidence or sophistry ; it 

 is an expression of the simplicity and order that are 

 fundamental attributes of the Universal Cosmos. 



