464 THE HISTORY OF BIOLOGY 



confessing a murder) immutable." Lamarck's theory of the modification of 

 species, however, Darwin was unable to accept; it appeared to him to be 

 "rubbish" — "Heaven forfend me from Lamarck's nonsense of 'a tendency 

 to progression. ' " Nor indeed in any other biological literature accessible to 

 him could he find any way out of the difficulty involved in the origin of 

 species. 



Darwin s experiments to -prove the mutability of species 

 During this period he was closely associated with Lyell, the scientist who 

 most influenced him — he too, as we have seen, no friend of Lamarck — 

 and resolved to deal with the species as Lyell had dealt with the geological 

 strata of the earth — namely, to collect as many facts as possible regarding 

 the transition from one form to another. In this respect domestic animals 

 seemed to him to give the best suggestions : that each separate domestic animal 

 was a true species no systematist had ever denied and it was likewise ac- 

 knowledged that man had produced a mass of different forms of every species 

 of that kind. Darwin placed himself in communication with a great many 

 animal-breeders, and himself for years bred different races of pigeons, all 

 for the purpose of discovering how the different races arose. Expert breeders 

 believed that by a selection of suitable parents it was possible gradually to 

 modify the progeny at will. Darwin also came to accept this view; all the 

 young in a litter of domestic animals are indeed somewhat unlike one another 

 and their parents — they "vary" as he says — and by selecting the suitable 

 variations it is possible to guide the breed in the required direction. But if 

 man was able by selection to produce out of the uniform canine type that 

 still exists among wild tribes such a large quantity of different forms, should 

 it not then be possible for species to be modified by nature in the same way? 

 The difference between a greyhound and a bulldog is far greater than that 

 between many wild life-forms which without doubt pass for good species. 

 But is there in nature a force operating in the same direction as the breeder 

 when he selects new forms of domestic animals? Here lay the worst stum- 

 bling-block. Then Darwin happened to read Malthus's above-mentioned 

 work on population : how both in nature and in human life there are produced 

 individuals in far greater numbers than there are means for maintaining, and 

 how the weakest perish in the competition for food. This gave him his idea; 

 in the struggle for existence those life-forms are destroyed that are least ca- 

 pable of adapting themselves to prevailing conditions, while the strongest 

 individuals survive and reproduce those qualities that have a greater chance 

 of survival. Thus the external conditions themselves come to multiply the 

 differences brought about by the variability of the offspring in relation to 

 the parents, until new varieties and new species arise. Consequently, the 

 struggle for existence induces a natural selection that operates similarly to 

 the choice of races among domestic animals exercised by man, only with 



