120 Animal Life and Intelligence. 



disappearing. Hence, in parasites and some forms of life 

 which live under simple conditions, we have the phenomena 

 of degeneration, or a passage from a more complex to a 

 more simple condition. 



Eevolution in organic life is the destruction of one 

 organism or group of organisms, and the replacement in 

 its stead of a wholly different organism or group of 

 organisms. During hard times there may be much revo- 

 lution, or replacement of one set of organic forms by 

 another set of organic forms. It was by revolution that 

 the dominant reptiles of the Mesozoic epoch were replaced 

 by the dominant mammals of Kainozoic times. It was by 

 revolution that pterodactyls were supplanted by birds. 

 Eevolution has exterminated many a group in geological 

 ages. On the other hand, it was by evolution that the 

 little-specialized Eocene ungulates gave rise to the horse, 

 the camel, and the deer ; by divergent evolution that the 

 bears and dogs were derived from common ancestors. 

 Palaeontology testifies both to evolution and revolution.* 

 That history does the same, I need not stay to exemplify. 

 The same laws also apply to systems of thought. Dar- 

 winism has revolutionized our conceptions of nature. 

 Darwin placed upon a satisfactory basis a new order of 

 interpretation of the organic world. By it other interpre- 

 tations have been supplanted. And now this new concep- 

 tion is undergoing evolution, not without some divergence. 



In this chapter we have seen how evolution is possible 

 under natural conditions. If the law of increase be true, 

 if more are born than can survive to procreate their kind, 

 natural selection is a logical necessity. We must not 

 blame our forefathers for not seeing this. Until geology 

 had extended our conception of time, no such conclusions 

 could be drawn. If organisms have existed but six or 

 seven thousand years, and if in the last thousand years 

 little or no change in organic life has occurred, the 

 supposition that they could have originated by any such 



* For evidence in copious abundance, see Nicholson's "Manual of 

 Palaeontology ," new edition, vol. i. : " Yertebrata," by E. Lydekker. 



