212 THE DESCENT OF MAN. Part I. 



kingdom of the Vertebrata it has culminated in man. 

 It must not, however, be supposed that groups of organic 

 beings are always supplanted and disappear as soon as 

 they have given birth to other and more perfect groups. 

 The latter, though victorious over their predecessors, 

 may not have become better adapted for all places in 

 the economy of nature. Some old forms appear to have 

 survived from inhabiting protected sites, where they 

 have not been exposed to very severe competition ; and 

 these often aid us in constructing our genealogies, 

 by giving us a fair idea of former and lost populations. 

 But we must not fall into the error of looking at the 

 existing members of any lowly-organised group as per- 

 fect representatives of their ancient predecessors. 



The most ancient progenitors in the kingdom of the 

 Vertebrata, at which we are able to obtain an obscure 

 glance, apparently consisted of a group of marine animals, 27 

 resembling the larvae of existing Ascidians. These 

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

 organised as the lancelet ; and from these the Ganoids, 

 and other fishes like the Lepidosiren, must have been 

 developed. From such fish a very small advance would 



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, — 

 supplied 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 period 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. 



