212 
THE DESCENT OP 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 
Vertebrate, 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 woidd 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. 
