I70 DISEASE IN PLANTS. 



a marked advance. However rough the experi- 

 ments may of necessity have to be, it is only by 

 such means that data can be gradually accumulated. 

 Having now obtained some insight into the 

 factors concerned in disease, let us enquire further 

 into the bearing of variation on these. It is 

 evident that pathological conditions may vary ; 

 indeed they are themselves symptoms of variation, 

 as we have seen. The history of all our cultivated 

 plants shows abundantly that many of the variations 

 obtained by breeding in our gardens, orchards, 

 fields, etc., involve differences of response on the 

 part of the plant to the very agencies which induce 

 disease. Every year the florists' catalogues offer 

 new " hardy " varieties ; but a hardy variety is 

 simply, for our present purpose, one which suc- 

 cumbs less readily to frost, cutting winds, cold 

 damp weather, and so forth. If anyone doubts 

 that hardy varieties have been gradually bred by 

 selection, I refer him to the evidence collected by 

 De Candolle, Darwin, Wallace, Bailey and others. 

 When we come to enquire into the causes of 

 " hardiness," however, difficulties at once beset us. 

 The adaptation may express itself in a difference 

 in the time of flowering or leafing, the exigencies 

 of the season being " dodged," as it were, in a 

 manner which was impossible with the original 

 stock, as appears to have occurred with Peaches in 

 America ; or it may be expressed in deeper rooting, 

 as is said to be the case in some Apples, or in the 

 acquirement of a more deciduous habit, or in 

 actually increased resistance to low temperatures. 



