2 PLANT-BREEDING 



and Darwin collected all the evidence concerning it which 

 he could find. The rapid victory gained by his views has 

 been due mainly to his discussion of this minor point. 



Direct observations concerning the first appearance of 

 species in nature were not at hand. In agriculture and in 

 horticulture, however, numerous observations had been made, 

 and for a number of races and varieties the origin was his- 

 torically known. Distinct methods were in use to guide 

 these changes and to produce varieties which would comply 

 with the demands of practice. The grand principle of all 

 these methods was selection. Selection means guiding the 

 changes in the specific characters of organisms by cutting 

 off all those which are changing in undesirable ways, and 

 reserving for reproduction only those which differ advan- 

 tageously from the average. 



Darwin proved that the origin of species in nature must 

 be the same phenomenon as the origin of races and varieties 

 in culture. He showed that in nature an analogous process 

 of selection is steadily active. More seeds are produced 

 and more children are born than can possibly survive, and 

 the decision as to which are to live and which must die de- 

 pends, on one side, on the life conditions and, on the other, 

 on the distinctive qualities of the competing individuals. Of 

 course, in the single instances survival depends mainly on 

 chance, but in the long run the different chances may be 

 assumed to annul one another's influence, and the decision 

 falls to individual excellences and life conditions. In this 

 way the latter can be said to make a choice of the individuals 

 best fitted for the local conditions and this is what is now 

 universally known as the principle oj natural selection. 

 It guides evolution, keeping it in the useful ways, and des- 

 troys all that try to diverge in opposite directions. 



The theory of common descent is Darwin's" theory, since 

 it has been founded by him on so broad a basis of facts as to 



