152 HEEBDITY AND SELECTION IN SOCIOLOGY 



cycle of evolution and dissolution which the individual organism 

 exhibits. 



Although it is incontestably true that every species must 

 partake of the fate of the world in toto, and, along with this 

 globe on which they live, one day vanish, as so many myriads of 

 other worlds have done, in the boundless space and time in which 

 the cosmos is plunged ; yet there seems no reason for affirming 

 that a species must pass through a cycle of evolution and disso- 

 lution owing to some inherent necessity of the germ-plasm. It 

 has, indeed, been affirmed that the intragerminal force which 

 brings about the evolution of species brings about their disso- 

 lution also ; and, if we accept Nageli's theory of the species as 

 a resultant of internal forces only, there is nothing to prevent 

 our supposing that the mysterious vital force does indeed 

 cause the regression and death of species, as well as their birth. 

 There is, however, no justification for postulating this force, 

 nor, indeed, any " inherent vital principle " independent of 

 selection ; such mystical principles lie outside the domain of 

 science. 



If species were indeed subject to a fatal law of evolution 

 and dissolution, there is no reason why the extinction of some 

 species should ensue so much sooner than that of others. Among 

 the Cephalopoda, the Nautilus has certainly existed since the 

 Silurian epoch ; on the other hand, we know that other species 

 have had a far shorter existence. Neumayer has shown that 

 palseontological evidence all goes to prove this great inequaUty 

 in the longevity of species, an inequality which is far too marked 

 to warrant the idea of a physiological death of species similar 

 to the physiological death of individuals. At the same time, 

 this great inequahty gives us the clue to the real reason for the 

 extinction of species — namely, inability to adapt themselves. 



The case of the extinct order of Trilobites illustrates what 

 we mean. The Trilobites seem to have been exterminated in 

 the Silurian epoch, precisely at the time when the various 



