io6 INVERTEBRATE PALAEONTOLOGY 



its acme, and then descends along a course reminiscent 

 of that by which it rose, dying of old age after a phase 

 of second-childhood, when its vitality is exhausted. In 

 the other case, over-specialization continues to produce 

 effects which, after the acme has been passed, become 

 positively disadvantageous in hypertrophy, however 

 profitable they may have been in restrained develop- 

 ment. But this "suicidal" tendency is none the less 

 comparable with phenomena often shown in individual 

 senescence. Morbid growths, either in bulk or sclerosis, 

 frequently precede, and induce, death; so that the 

 analogy between racial and individual life is still 

 tenable. 



(IV) ACCELERATION AND RETARDATION 



The rate at which specialization proceeds varies to a 

 marked degree in different groups of animals, and is not 

 constant during the evolution of a single series ; it may 

 differ in the several parts of an " individual " organism. 

 Some stocks seem born to stagnation, others to progress. 

 The racial history of the former types tends to be long 

 and uneventful, that of the latter short and crowded 

 with morphogenetic episode. The contrast could hardly 

 be displayed more clearly than in the case of the 

 Tetrabranchiate Cephalopods already discussed. The 

 Nautiloids, like the Ammonites, originated as forms 

 with straight, chambered shells which have shown 

 a steady tendency towards enrolment from Lower 

 Palaeozoic times onwards. But whereas the Ammonoids 

 attained the acme of their specialization in the Trias? 

 and hastened to destruction in the Cretaceous period, 

 the Nautiloids have not yet reached a degree of elabora- 

 tion comparable with that of an Ammonite, and show no 

 signs of aspiration to such an ideal. Although some 



