458 THE MULTIPLICATION OF EFFECTS. 



in some cases, be led into modes of defence or escape differ- 

 ing' 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 organiza- 

 tion. Observe now, however, a further consequence. 

 There must arise not simply a tendency towards the differ- 

 entiation 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 alterations quite indefinite in 

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

 stitute an advance. Probably in most cases the modified 

 type will be not appreciably more heterogeneous than the 

 original one. But it must now and then occur, that some 

 division of a species, falling into circumstances which give 

 it rather more complex experiences, and demand actions 

 somewhat more involved, will have certain of its organs 

 further differentiated in proportionately small degrees 

 will become slightly more heterogeneous. Hence, there will 

 from time to time arise an increased heterogeneity both of 

 the Earth's flora and fauna, and of individual races included 

 in them. Omitting detailed explanations, and allowing for 

 the qualifications which cannot here be specified, it is suffi- 

 ciently clear that geological mutations have all along tended 

 to complicate the forms of life, whether regarded separately 

 or collectively. That multiplication of effects which has 

 been a part-cause of the transformation of the Earth's 

 crust from the simple into the complex, has simultane- 

 ously led to a parallel transformation of the Life upon its 

 surface.* 



* Had this paragraph, first published in the Westminster Review in 1857, 

 been written after the appearance of Mr. Darwin's work on The Origin of 





