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^rtsibtiitiai ^bbri:ss* 



By Prof. C. LLOYD MORGAN. 



THE NATURE AND ORIGIN OF VARIATIONS. 



NATURAL selection is dependent upon (1) the law of 

 increase, or the tendency of every known species to 

 increase in nnmbers ; (2) the struggle for existence, by which 

 the tendency to increase is held in check ; and (3) the occur- 

 rence oi' variations. In the absence of variation those 

 individuals which succumb in the struggle for existence 

 would be similar to those which survive. The numbers 

 w^ould indeed be reduced, and the tendency to increase 

 would be held in check ; but the species would remain un- 

 changed. Given, however, the occurrence of variations in the 

 offspring, the struggle for existence w^ill act differentially. 

 Those which fall below the standard of mediocrity will be 

 eliminated, those which reach this standard or excel it will 

 survive to procreate their kind. We may express this 

 roughly and diagrammatically thus :— 



a-^ — m'' etc. 

 a^ — m' h^ 

 a3 — m * b' 

 a^ — m^ b'' 

 ai — m2 b^^ 

 a — m^ b- 

 in b^ 

 b 



The letters m, in\ etc., stand for the mean or average of 



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