EXTENSION OF EVOLUTIONARY THEORIES 347 



Hence the multiplicity of our actual species cannot 

 be due to mutations. What can we say to de Vries's 

 objections to variations ? 



It has been rightly pointed out that these objections 

 are unsound. In the first place, we have really pro- 

 duced new races of dogs and pigeons by artificial 

 selection, and these have retained their new characters 

 when crossed with their own kind. We may therefore 

 assume that in natural selection also the modified 

 animals will retain their characteristics when the 

 selection has ceased. But even that is not necessary. 

 Selection does not cease to act even in pronounced 

 species ; it watches unceasingly over the animals, and 

 continues to act on them. Hence the variations, even 

 if they had a tendency constantly to slip back, must 

 lead to new species, as natural selection always cuts 

 off all instances of reversion. 



Secondly, even if after a time we reach a limit that 

 we cannot pass in artificial selection, it does not follow 

 that this is the case in nature as well. We can only 

 continue to select as long as the harmony of the various 

 parts is preserved, and we do not know what other parts 

 of the body we must attend to in our selection if the 

 object we are breeding is to have unusual features. If 

 we want to increase the gooseberry to the size of a 

 pumpkin, it is certainly not enough to select the trees 

 that have the largest berries. If we want very large 

 berries we must also have changes in the plant's fibres 

 for conveying water and sap, the branches must be 

 thicker in a word, there must be a number of changes 

 that are not entirely within our knowledge or control. 



