132 EELATIVE LONGEVITY OF SPECIES. [Ch. X. 



imply the co-existence of the same mammalia with man. The antiquity, 

 therefore, of the human race may be inferred from the concurrent 

 testimony of several independent classes of geological facts. In the 

 first place, the disappearance of many wild animals from a large con- 

 tinent, even where man has been an active agent of extermination, 

 must always require a considerable lapse of time for its accomplish- 

 ment ; indeed, before the invention of fire-arms, it is hard to say how 

 many centuries it would take to bring about such utter extirpation. Yet 

 there can be no doubt that many species became extinct after man 

 was a denizen of the earth, and before the Danish shell-mounds were 

 formed, or the oldest of the Swiss lake-dwellings constructed. Sec- 

 ondly, thousands of years must have been required to enable rivers to 

 deepen and widen their valleys, and to grind down fragments of rock 

 into mud, sand, and pebbles, on such a scale as to produce, the old 

 valley gravels, both higher and lower, containing flint implements and 

 the bones of extinct mammalia. Thirdly, much time is also demand- 

 ed to enable springs and engulfed rivers to change their courses, and 

 for caves which once lay in the line of a great subterranean drainage 

 to become dry, and to have their floors encrusted over with a hard 

 covering of stalagmite. Lastly, ages must have been required to 

 bring about such a change in the climate of a wide region as to cause 

 the winters to be less severe, and the geographical distribution of 

 certain species of mammalia and land and freshwater shells to vary. 

 The length of the historical epoch, even if assumed to be 3000 or 

 4000 years, does not furnish us with any appreciable measure for cal- 

 culating the number of centuries which would suffice for such a series 

 of changes, which are by no means of a local character, but have 

 already been traced from England and the North-west of France to 

 Sardinia and Sicily. 



Relative longevity of species in the mammalia and testacea. — I called 

 attention, in 1830,* to the fact which had not at that time attracted 

 notice, that the association in the post-pliocene deposits of shells, 

 exclusively of living species, with many extinct quadrupeds, beto- 

 kened a longevity of species in the testacea far exceeding that in the 

 mammalia. Subsequent researches seem to show that this greater 

 duration of the same specific forms in the class mollusca is dependent 

 on a still more general law, namely, that the lower the grade of ani- 

 mals, or the greater the simplicity of their structure, the more per- 

 sistent are they in general in their specific characters throughout vast 

 periods of time. Not only have the invertebrata, as shown by geo- 

 logical data, altered at a less rapid rate than the vertebrata, but if we 

 take one of the classes of the former, as for example the mollusca, we 

 find those of more simple structure to have varied at a slower rate 

 than those of a higher and more complex organization ; the brachio- 

 poda, for example, more slowly than the lamellibranchiate bivalves, 



* Principles of Geology, 1st ed. vol. iii. p. 140. 



