LECTUEE III 

 THE DARWINIAN THEORY (continued) 



Natural selection — Variation — Struggle for existence — Geometric ratio of rate of 

 increase — Normal number and ratio of elimination in a species — Accidental causes 

 of extinction — Dependence of the strength of a species on enemies — Struggle for 

 existence between individuals of the same species — Natural selection affects all organs 

 and stages — Summary. 



In artificial selection, through which, with or without conscious 

 intention, our domesticated animals and cultivated plants have arisen, 

 there must obviously be three kinds of co-operative factors : first, 

 the variability of the species ; second, the capacity of the organism 

 for transmitting its particular characters to its progeny; and third, 

 the breeder who selects particular qualities for breeding. No one of 

 the factors can be dispensed with ; the breeder could effect nothing, 

 were there not presented to him the variations of parts in the 

 particular direction in which he wishes them to vary; an indefi- 

 nite variation, that is, a variation not guided by selection, would never 

 lead to the formation of new breeds ; the species would probably 

 become in time a motley mixture of all sorts of variations, but 

 a breed with definite characters, transmissible in their purity to its 

 descendants, could never be formed. Finally, every process of selec- 

 tive breeding would be futile, if the variations which appeared could 

 not be transmitted. 



Darwin assumes that processes of transformation quite similar 

 to those which take place under the guidance of Man occur also in 

 nature, and that it is mainly these which have brought about and 

 guided the transformations of species which have taken place in 

 the course of the earth's history. This process he calls natural 

 selection. 



It will readily be admitted that two out of the three factors 

 necessary to a process of selective breeding are present also in the 

 natural conditions of the life of species. Variability in some degree 

 or other is absent from no species of animal or plant, though it may 

 be greater in one than in another, and it cannot be doubted that the 

 inborn differences which distinguish one individual from another are 

 capable of transmission. It is only to untrained observers that all the 

 individuals of a species appear alike ; for instance, all garden whites, 

 or all the individuals of the small tortoiseshell butterfly (Vanessa 



