CHAPTEE VII 



HEREDITY, VARIATION, INCREASE 



In the preceding chapters I have shown how, from a con- 

 sideration of the simple facts of the numerical distribution 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 strug- 

 gle 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 (11th edition, 18 68, but w^hich first appeared in the 

 9th 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 in- 

 terest even by the evolutionist of to-daj^ 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 eifect had been hitherto unperceived and 

 neglected. The two first of these factors we will now pro- 

 ceed to discuss and elucidate. 



Perhaps the most universal fact — sometimes termed ^' law " 

 — of the organic world is, that like produces like — that off- 



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