72 THE LESSON OF EVOLUTION 



feed on sea-weeds, the others on animals ; so we may 

 safely infer that the ancient species had similar 

 habits. The presence of eyes in Trilobites is suffi- 

 cient proof that they had optic nerves , a brain , and 

 a fairly well organised nervous system. And we 

 know that all must have had reproductive systems. 

 Indeed there is no known extinct animal or plant 

 which cannot be referred to some class which has 

 living representatives, and which, consequently, 

 must have had the same type of structure. 



2. Every extinct species has a limited duration 

 in time, and it is the same with extinct genera, 

 families, and orders. This generalisation depends 

 largely on negative evidence, yet this evidence points 

 so uniformly in the same direction that it has been 

 unhesitatingly accepted. 



But although every extinct species or genus had a 

 limited life, the duration of specific or generic life 

 differs much in different forms. Of course, as a 

 general rule, the life of a genus is longer than that 

 of a species, because a genus contain several species, 

 but this is not always the case. In the Ammonoidea 

 there are several families which are limited to a 

 single period, as also are some orders of Eeptilia, 

 and even a sub-class in the birds. Usually we find 

 that the lowly organised species endured longer than 

 the more specialised forms. Some of the Protozoa 

 -of the lower Palaeozoic are with difficulty separated 

 from living forms ; and Lingula , a low form of 

 Brachiopod, may also be cited in illustration. But 

 this is not always, the case, as, for example, Num- 

 mulites and Graptolites which had a comparatively 



