204 THE DESCENT OF MAN. [Part I. 



glance, apparently consisted of a group of marine ani- 

 mals, 27 resembling the larvae of existing Ascidians. These 

 animals probably gave rise to a group of fishes, as lowly 

 organized as the lancelet ; and from these the Ganoids, 

 and other fishes like the Lepidosiren, must have been de- 

 veloped. From such fish a very small advance would 

 carry us on to the amphibians. We have seen that birds 

 and reptiles were once intimately connected together ; and 

 the Monotremata now, in a slight degree, connect mam- 

 mals with reptiles. But no one can at present say by 

 what line of descent the three higher and related classes, 

 namely, mammals, birds, and reptiles, were derived from 

 either of the two lower vertebrate classes, namely am- 

 phibians and fishes. In the class of mammals the steps 

 are not difficult to conceive which led from the ancient 

 Monotremata to the ancient Marsupials ; and from these 

 to the early progenitors of the placental mammals. We 

 may thus ascend to the Lemuridse ; and the interval is not 

 wide from these to the Simiadaa. The Simiadse then 

 branched off into two great stems, the New World and 

 Old World monkeys ; and from the latter, at a remote 



27 All vital functions tend to run their course in fixed and recurrent 

 periods, and with tidal animals the periods would probably be lunar ; for 

 such animals must have been left dry or covered deep with water — sup- 

 plied with copious food or stinted — during endless generations, at regular 

 lunar intervals. If, then, the vertebrata are descended from an animal 

 allied to the existing tidal Ascidians, the mysterious fact that, with the 

 higher and now terrestrial Vertebrata, not to mention other classes, many 

 normal and abnormal vital processes run their course according to lunar 

 periods, is rendered intelligible. A recurrent period, if approximately 

 of the right duration, when once gained, would not, as far as we can 

 judge, be liable to be changed ; consequently it might be thus transmitted 

 during almost any number of generations. This conclusion, if it could 

 be proved sound, would be curious ; for we should then see that the pe- 

 riod of gestation in each mammal, and the hatching of each bird's eggs, 

 and many other vital processes, still betrayed the primordial birthplace 

 of these animals. 



