BIOLOGICAL PALAEONTOLOGY 115 



There are four crises in the life of most animals, 

 particularly of the Metazoa. The first, that of fertiliza- 

 tion of the ovum, may be claimed as almost u creative " 

 in effect ; but its significance is far from clear, and the 

 phenomena of parthenogenesis show that it is not 

 universally necessary. The second crisis occurs after a 

 period of larval or embryonic development, when meta- 

 morphosis, or birth, is reached. This normally takes 

 place when ontogeny has produced some approximation 

 to adult structure. The third crisis marks arrival at 

 maturity, the fourth is death. In comparison with race- 

 life, individual " birth " may be taken to represent the 

 inception of a new stock, maturity corresponds with the 

 acme, and death suggests extinction. By analogy, it is 

 pardonable to speculate that fertilization is a " recapitula- 

 tion " of the origin of life. 



The crises are of relatively short duration ; but they 

 are clearly recognizable in phylogeny in all cases where 

 reasonably complete knowledge is available. They serve 

 as convenient divisions in developmental sequence of 

 individual or group-life. The intervals between the 

 crises vary much in length in different types of Metazoa ; 

 their duration is not necessarily commensurate with the 

 amount of ontogenetic development attained. There 

 is, however, a tendency for more highly specialized 

 organisms to require longer periods for growth ; though 

 the phyletic " age " of the stock seems to have some 

 influence. 



The larval (or embryonic) stage of development is so 

 clearly connected with phyletic evolution that it needs 

 no further emphasis in this place. It is in the later 

 phases of individual growth that comparisons with 

 phylogeny are more feasible, in view of the nature of 

 palaeontological evidence. Between birth and maturity, 

 three stages in " anagenesis " can be arbitrarily separated 



