OLD AND NEW THEORIES COMPARED. 341 



quite alive to the effects of the geometrical ratio of 

 increase, and of the struggle for existence which thence 

 inevitably follows. 



Mr. Darwin, on the other hand, comparatively in- 

 different to, or at any rate silent concerning the causes 

 of those variations which appeared so all-important to 

 Lamarck, inasmuch as they are the raindrops which 

 unite to form the full stream of modification, goes 

 into very full detail upon natural selection, or [the 

 survival of the fittest, and maintains it to have been 

 "the most important but not the exclusive means of 

 modification." * 



It will be readily seen that, according to Lamarck, 

 the variations which when accumulated amount to 

 specific and generic differences, will have been due to 

 causes which have been mainly of the same kind for 

 long periods together. Conditions of life change for 

 the most part slowly, steadily, and in a set direction ; 

 as in the direction of steady, gradual increase or 

 decrease of cold or moisture ; of the steady, gradual 

 increase of such and such an enemy, or decrease of 

 such and such a kind of food ; of the gradual upheaval 

 or submergence of such and such a continent, and con- 

 sequent drying up or encroachment of such and such a 

 sea, and so forth. The thoughts of the creature varying 

 will thus have been turned mainly in one direction for 

 long together ; and hence the consequent modifications 

 will also be mainly in fixed and definite directions for 

 many successive generations ; as in the direction of a 

 warmer or cooler covering ; of a better means of defence 



* ' Origin of Species,' p. 4. 



