CHAPTER VII 



HEREDITY, VARIATION, INCREASE 



IN the preceding chapters I have shown how, from a 

 consideration of the simple facts of the numerical distribu- 

 tion of species over the earth, together with the varying 

 numbers of the individuals in each species and the area 

 occupied by them, we are led to the conclusion that there 

 is an ever-present struggle for existence between species 

 and species, resulting in a continual readjustment to the 

 environment. In this view there is no question of any 

 change of species, but merely of their redistribution ; we 

 perceive that during the process very rare or local species 

 may, and certainly do, die out, but we have obtained no 

 clue to the method by which new species arise to replace 

 them. 



This was the state of opinion among the most advanced 

 writers before Darwin, and it is very clearly expressed in 

 the admirable 42nd chapter of Sir Charles Lyell's Principles 

 of Geology (iith edition, 1868, but which first appeared 

 in the pth edition, 1853, pp. 689-701) many years before 

 the idea of the transmutation of species had been seriously 

 entertained by men of science. This chapter may still be 

 read with interest even by the evolutionist of to-day. The 

 reader will then be better able to appreciate the enormous 

 advance made by Darwin by his conception of " natural 

 selection," dependent on the three fundamental factors 

 heredity, variation, and enormous powers of increase all 

 well known to naturalists, but whose combined effect had 

 been hitherto unperceived and neglected. The two first of 

 these factors we will now proceed to discuss and elucidate. 



Perhaps the most universal fact sometimes termed 



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