48 progress: its law antd cause. 



would there be certain modifications tlius caused by change 

 of physical conditions and food, but also in some cases 

 other modifications caused by change of habit. The fauna 

 of each island, peopling, step by step, the newly-raised 

 tracts, would eventually come in contact with the fliunas 

 of other islands ; and some members of these other fiiunas 

 would be unlike any creatures before seen. Herbivores 

 meeting with new beasts of prey, would, in some cases, be 

 led into modes of defence or escape diflfering from those 

 previously used ; and simultaneously the beasts of prey 

 would modify their modes of pursuit and attack. We 

 know that when circumstances demand it, such changes of 

 habit do take place in animals ; and we know that if the 

 new habits become the dominant ones, they must eventually 

 in some degree alter the organization. 



Observe, now, however, a further consequence. There 

 must arise not simply a tendency towards the diflTerentia- 

 tion of each race of organisms into several races ; but also 

 a tendency to the occasional production of a somewhat 

 higher organism. Taken in the mass these divergent varie- 

 ties which have been caused by fresh physical conditions 

 and habits of life, will exhibit changes quite indefinite in 

 kind and degree ; and changes that do not necessarily con- 

 stitute an advance. Probably in most cases the modified 

 type will be neither more nor less heterogeneous than the 

 original one. In some cases the habits of life adopted 

 being simpler than before, a less heterogeneous structure 

 will result : there will be a retrogradation. But it must 

 now and then occur, that some division of a species, falling 

 into circumstances which give it rather more complex expe- 

 riences, and demand actions somewhat more involved, will 

 have 'certain of its organs further differentiated in propor- 

 tionately small degrees, — will become slightly more hetero- 

 geneous. 



Thus, in the natural coui'se of things, there will from 



