158 THE THEORY OF EVOLUTION 



nature in order to produce ever better adapted and 

 higher forms. 



Since in nature man does not, as in breeding 

 experiments, supervise the selection and thereby fix 

 a desired change, he consequently sought another 

 selective factor, and found it in the so-called ' natural 

 selection/ He published his opinions in the well- 

 known book ' Origin of Species by Means of Natural 

 Selection : or, the Preservation of Formed Races in the 

 Struggle for Life' (1859), and 'The Descent of Man 5 

 (1871). 



The progeny of the same parents are, according 

 to him, never perfectly like the parents or each other : 

 there are always differences, favourable or unfavourable, 

 among the individuals concerned. All the progeny 

 cannot survive : if we think of the thousands of seeds 

 which a single plant can produce, only those which, 

 by chance, are more favourably constituted have a 

 prospect of preservation ; they alone succeed also in 

 reproduction. The favourable variations, furthermore, 

 are transmitted. Through some of the offspring of 

 these already better adapted individuals there occurs, 

 again absolutely by chance, an increase of the favouring 

 peculiarities ; and so it continues. 



It is seen (a) that the first original and each separate 

 increase of a favourable change as regards struggle 

 for existence happens by pure chance ; there occur also 

 at least as many unfavourable ones ; (b) the favoured 

 individuals become preserved because they alone survive 

 and reproduce themselves ; (c) the characters which 



