THE AGE OF INVERTEBRATES OF THE SEA. 85 





these forms bad become predominant. We shall also see that 

 just as the leading groups of Mollusks and Crustaceans seem 

 to have had no ancestors, so it is with the groups of 

 Vertebrates which take their places. It is also interesting to 

 observe that already in the Palaeozoic all the types of inverte- 

 brate marine life were as fully represented as at present, and 

 that this swarming marine life breaks upon us in successive 

 waves as we proceed upward from the Cambrian. Thus the 

 progress of life is not gradual, but intermittent, and consists in 

 the sudden and rapid influx of new forms destined to increase 





'. ':■■: ... -i 



^''^ ' I// 



Fig. 81. — Amphif<eltis paradoxus (Salter). 

 A Devonian Svomapud. 



Fir,. 82. — Anthropalcemon Hilliana 

 (Dn ). A Carhoniferuus Decapod. 

 The carapace only. . 



and multiply in the place of those which are becoming effete 

 and ready to vanish away or to sink to a lower place. Farther, 

 since the great waves of aquatic life roll in with each great 

 subsidence of the land, a fact vvhich coincides with their 

 appearance in the limestones of the successive periods, it 

 follo'vs that it is not struggle for existence, but expansion under 

 favourable circumstances and the opening up of new fields of 

 migration that is favourable to ihe introduction of new species. 

 The testimony of palaeontology on this point, which I have 



