THE COMING OF EVOLUTION 211 



external conditions upon the variations from the 

 specific type which individuals present and which 

 we call ' spontaneous ' because we are ignorant of 

 their causation is as wholly unknown to the historian 

 of scientific ideas as it was to biological specialists 

 before 1858. But that suggestion is the central idea 

 of the Origin of Species and contains the quint- 

 essence of Darwinism." 



With this idea as a working hypothesis, Darwin 

 spent twenty years collecting facts and making ex- 

 periments. He read books of travel and treatises on 

 sport, natural history, horticulture, and the breeding 

 of domesticated animals. He carried out experiments 

 on the crossing of tame pigeons ; he studied the trans- 

 port of seeds, and the geological and geographical 

 distribution of plants and animals. In the assimila- 

 tion of facts, in appreciating their bearing on all the 

 complicated questions which arose, and in marshalling 

 them at the last, Darwin showed himself supreme. 

 His transparent honesty, burning love of truth, and 

 calm and even balance of mind, form a model of the 

 ideal naturalist. Fertile in hypotheses as a guide to 

 work, he never let a preconceived view blind him to 

 facts. " I have steadily endeavoured," he writes, " to 

 keep my mind free so as to give up any hypothesis, 

 however much beloved (and I cannot resist forming 

 one on every subject), as soon as facts are shown to be 

 opposed to it." 



By 1844 Darwin had convinced himself that species 

 were not immutable, but still he worked on year after 

 year to gain yet surer evidence. In 1856 Lyell urged 

 him to publish the results of his researches ; Darwin, 

 not satisfied with their completeness, delayed. On June 



