452 COMPARATIVE ZOOLOGY 



tions, was propounded by Charles Darwin (1809-1882) 

 in his " Origin of Species by Means of Natural Selec- 

 tion, or the Preservation of Favored Races in the Strug- 

 gle for Life," published in 1859. Darwin had served as 

 naturalist on the British exploring ship Beagle on a five 

 years' cruise (1832-1837) around the world, and "was 

 much struck with certain facts in the distribution of the 

 organic beings inhabiting South America, and in the 

 geological relations of the present to the past inhabit- 

 ants of that continent." After his return home, twenty 

 additional years were spent in collecting facts, making 

 further observations and experiments, and in pondering 

 the theory before he ventured to publish his results and 

 to state what he regarded as the factors concerned in 

 the process of evolution. A similar conclusion had been 

 reached, simultaneously and independently, by Alfred 

 Russell Wallace (1823- ), who had travelled exten- 

 sively in South America and the Malay Archipelago, 

 and, like Darwin, had become convinced of the certainty 

 of evolution, and sought for its.explanation. 



^As held by Darwin, the theory of evolution, together 

 with the causes of the process, may be briefly stated as 

 follows : 



(i) Organisms tend to produce a great many more off- 

 spring than can stirvive. Linnaeus showed that the 

 number of living descendants of an annual plant which 

 produced only two seeds each year would, at the end of 

 twenty years, be over a million. There is, however, no 

 plant known to be so unproductive. With reference to 

 the elephant, regarded as the slowest of breeders, pro- 

 ducing at the age of thirty a pair of young, and a pair 

 every thirty years thereafter, and living to be one hundred 

 years old, Darwin computed that at the end of 750 years 

 there would be about nineteen million living elephants 

 all descended from the first pair. Individual insects lay 



