COURSE OF DEVELOPMENT ON THE EARTH, 149 



quired to obtain their food. It is obvious 

 that his remarks will also apply to the boas, 

 pythons, and great tortoises. 



We thus perceive that the Human Period is 

 the natural successor of those which have pre- 

 ceded it. The discoveries of science have long 

 been breaking down the barrier w r hich was 

 formerly supposed to exist between the physical 

 nature of man and that of the lower animals. 

 But continued observations have also been 

 breaking down the barrier between Instinct and 

 Reason, which was once deemed still more in- 

 surmountable ; and it is now no longer possible 

 for man to arrogate to himself the exclusive 

 privilege of immortality. 



There is reason to believe that the social 

 Hymenoptera, and especially ants, have attained 

 to a high degree of such civilisation as is con- 

 sistent with their nature. They recognise ants 

 belonging to the same nest after months of 

 absence, although they instantly attack a 

 stranger ; and some species keep cattle (aphides, 

 etc.) ; others, slaves (ants of a weaker species 

 than their own, who have been carried off on 

 predatory excursions); while others devote the 

 greater portion of their time to agriculture, 



