No. m] PROGRESSION OF LIFE IN THE SEA 499 



and cucumarian. Tornaria developed into Balanoglos- 

 sus, whose structure hints to us that the ascidians and 

 vertebrates came from a similar stock. All the phyla 

 thus represented derive directly from the free-swimming 

 Ctenophore-like ancestor, and only one considerable 

 group, the Arthropods, remains unaccounted for. The 

 evolutionary history of an Arthropod is, however, not in 

 doubt. Its marine representatives, the Trilobites and 

 Crustacea, came directly from annelids, which, after their 

 desertion of a pelagic life to burrow in the sea-floor and 

 run along its surface, again took to swimming, and not 

 only stocked the whole mass of the water with a rich and 

 varied life of Copepods, Cladocera and Schizopods, but 

 gave rise to Amphipods, Isopods, and Decapods, groups 

 equally at home when roaming on the bottom or swim- 

 ming above it. 



Another important addition to the pelagic fauna we 

 should also notice here. From the molluscs, creeping on 

 solid surfaces, sprang a group of swimmers, the Cephalo- 

 pods, which have grown to sizes almost unequaled 

 amongst the animals of the sea. 



All these invertebrate phyla had become established 

 and most of them had reached a high degree of develop- 

 ment in the seas of Cambrian times. Amongst animals 

 then living there are many which have survived with little 

 change of form until to-day. One is almost tempted to 

 suggest that the life which the sea itself could produce 

 was then reaching its summit and becoming stabilized. 

 Since Cambrian times geologists tell us some thirty mil- 

 lion years ^° have passed, a stretch of time which it is 

 really difficult for our imaginations to picture. During 

 that time a change of immense moment has happened to 

 the life of the sea ; but if we read the signs aright, that 

 change had its origin rather in an invasion from without 

 than in an evolution from within. Whence came that 

 tribe of fishes which now dominates the fauna of the 

 sea! It would be rash to say that we can give any but a 

 speculative reply to the question, but the probable an- 

 swer seems to be that fishes were first evolved not to meet 



