1(]0 THE DESCENT OP MAN. 



iting 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 popula- 

 tions. But we must not fall into the error of looking at the exist- 

 ing members of any lowly-organized group as perfect representa- 

 tives 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,»= 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 developed. From such flsh 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 Mono- 

 tremata now connect mammals with reptiles in a slight degree. 

 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 the two lower vertebrate classes, namely, 

 amphibians 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 progeni- 



"^ The Inhabitants of the seashore must be greatly affected by the 

 tides; animals living either about the mean high-water mark, or 

 about the mean low-water mark, pass through a complete cycle of 

 tidal changes in a fortnight. Consequently, their food supply will un- 

 dergo marked changes week by week. The vital functions of such 

 animals, living under these conditions for many generations, can 

 hardly fail to run their course in regular weekly periods. Now it is 

 a mysterious fact that in the higher and now terrestrial Vertebrata, 

 as well as in other classes, many normal and abnormal processes have 

 one or more whole weeks as their periods; this would be rendered 

 intelligible if the Vertebrata are descended from an animal allied to 

 the existing tidal Ascidians. Many instances of such periodic proc- 

 esses might be given, as the gestation of mammals, the duration of 

 fevers, &0. The hatching of eggs affords also a good example, for, 

 according to Mr. Bartlett ('Land and Water,' Jan. 7, 1871), the eggs 

 of the pigeon are hatched in two weeks; those of the fowl in three; 

 those of the duck in four; those of the goose in five; and those of the 

 ostrich in seven weeks. As far as we can Judge, a recurrent period, 

 if approximately of the right duration for any process or function, 

 would not, when once gained, be liable to change; conseqiiently It 

 might be thus transmitted through almost any number of generations. 

 But if the function changed, the period would have to change, and 

 would be apt to change almost abruptly by a whole week. This con- 

 clusion, if sound, is highly remarkable; for the period of gestation in 

 each mammal, and the hatching of each bird's eggs, and many other 

 vital processes, thus betray to us the primordial birth-place of these 

 animals. 



