354 THE EVOLUTION THEORY 



no support for this view. I need not repeat his arguments, but will 

 simply refer to his clear and concise exposition of the problem. It is 

 obvious that our theory of the extinction of species as due to external 

 causes cannot be rejected on the ground that our knowledge of the 

 struggle that species had to maintain for their existence in past times 

 is even more imperfect than our knowledge of the struggle nowada^^s, 

 and that we are frequently unable to judge of it at all. But the facts 

 of geology are of value in another, quite different way. They reveal 

 such an extraordinary dissimilarity in the duration of species, and 

 also of the great groups of organisms, that the dissimilarity of itself 

 is sufficient to prevent our regarding the extinction of species as 

 regulated by internal causes. Certain genera of Echinoderms, such 

 as starfish (Astroj^eden), lived in the Silurian times, and they are 

 represented nowadays in our seas by a number of species ; and in the 

 same way the Cephalopod genus Nautihis has maintained itself 

 among the living all through the enormous period from the Silurian 

 sea to our own day. Formerly the Nautilids formed a predatory 

 horde that peopled the seas, and, as we have seen, we may perhaps 

 attribute to their dominance the disappearance of an order of 

 Crustaceans, the Trilobites, which were equally abundant at that 

 period. Now only a single species of nautilus (Nautihis j^ompillus) 

 lives on the coral reefs of the southern seas. Similarly, the genus 

 Lingula of the nearly extinct class of Brachiopods, somewhat mussel- 

 like sessile marine animal-s, has been preserved from the grey dawn 

 of primitive times, with its records in the oldest deposits, and is 

 represented in the living world of to-day by the so-called 'bar- 

 nacle-goose' mussel, Lingula anatina. 



On the other hand, we know of numerous species which lasted 

 for quite a short time, such as, for instance, the individual members 

 of the series of Steinheim Planorbis species, or of the Slavonic 

 Paludina species. Not infrequently, too, genera make their appear- 

 ance and disappear again within the period of one and the same 

 geological stratum. 



These facts not only tell against an unknown vitalistic principle 

 of evolution, but in general against the idea of the determination 

 of the great paths of evolution by purely internal causes. If there 

 were a principle of evolution the dissimilarity in the duration of life 

 could not be so excessive : if there were a ' senile stage ' of species 

 and a natural death of species comparable to the natural death of 

 individuals, it would not have been possible for most of the Nautilidse 

 to liave been restricted to the Silurian epoch, and yet for one species 

 to have continued to live till now ; and if there were a ' tendency ' 



