220 THE DESCENT OF MAN 



The most ancient progenitors in the kingdom of the Ver- 

 tebrata, at which we are able to obtain an obscure glance, 

 apparently consisted of a group of marine animals," resem- 

 bling the larvse of existing Ascidians. These animals proba- 

 bly 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 

 fish a very small advance would carry us on to the Am- 

 phibians. We have seen that birds and reptiles were once 

 intimately connected together; and the Monotremata 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 progenitors of the placental mammals. 

 "We may thus ascend to the Lemuridse; and the interval is 

 not very wide from these to the Simiadse. The Simiadae 



^^ The inhabitants of the sea-shore 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 undergo 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 processes might be given, as the gestation of 

 mammals, the duration of fevers, etc. The hatching of eggs affords also a good 

 example, for, according to Mr. Bartlett ("Land and "Water," Jan. 1, 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; consequently it might be thus transmitted thi;ough 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 conclusion, if sound, is highly remarkable ; for the period of gestation in 

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

 processes, thus betray to us the primordial birthplace of these animals. 



