VARIATION AND DISEASE. 173 



and sweet oranges the cultivator is selecting, and 

 intensifying by selection, very different metabolic 

 processes in the cell : he can test the results of 

 these, and so the selection proceeds. 



The question is. Could he select at the same 

 time those variations in cell activity which express 

 themselves in properties of the flower, fruit, foliage, 

 etc., he desires, as well as such variations as aid 

 the cells in repelling fungi, insects, or exigencies 

 of the non-living environment ? 



That more or less disease-proof varieties could 

 be selected if that object alone were kept in view 

 can hardly be doubted ; plenty of examples exist 

 already which show that the necessary variations 

 to work upon exist in just those secretions of 

 protoplasm, etc., which we have seen are concerned 

 in repelling or attracting parasites. 



The Sweet Almond has lost the power of pro- 

 ducing amygdalin and prussic acid in its cells ; 

 Cinchona plants vary immensely in the quantity 

 of quinine formed, and in European hot-houses 

 may even form none at all ; some varieties of 

 Maize have sugar and dextrine instead of starch 

 in their endosperms, or coloured instead of clear 

 sap in the aleurone layer, and recent researches 

 prove that they can transmit these peculiarities to 

 hybrid offspring ; non-poisonous bacteria have 

 frequently been got from poisonous species simply 

 by cultivation under special conditions, and pig- 

 mented forms can be bred into non-pigmented 

 races. 



But we see that the difficulty of selection is 



