342 DISEASES OF FIELD & GARDEN CROPS. [CH. 



Sometimes, after several healthy generations of plants, 

 the progeny may revert back to disease in an analogous 

 way with the many similar instances so well known 

 amongst men and other animals. The disease in these 

 cases is sometimes derived from some remote and perhaps 

 forgotten ancestor. 



A rotation of crops is of high importance as regards 

 disease, for a fungus that destroys turnips or cabbages 

 will probably not injure corn, and neither will injure 

 clover. As a rule any given destructive fungus keeps to 

 one Natural Order of plants, often to one genus, some- 

 times to one species. Any fungus capable of invading 

 plants belonging to several Natural Orders is an exception 

 to the rule. 



In writing these concluding words We are strongly 

 impressed by the fact of how little has really been described. 

 The mere margin of each subject has been barely ap- 

 proached, and the anatomy and physiology belonging to 

 each disease only glanced at. The subjects discussed have 

 only been presented in bare outline, and many of great im- 

 portance have not been mentioned at all as the diseases of 

 beans, beet, cucumbers, mint, hops, etc. A large subject 

 awaits description and illustration in the nature of canker 

 and the diseases of our fruit crops, our grapes, peaches, 

 nectarines, apples, pears, gooseberries, strawberries, figs, 

 melons, etc. Another of equal importance presents itself 

 in the diseases of our timber trees, our firs, pines, larches, 

 oaks, elms, and latterly, yews. 



In closing, we advise all students of nature to think as 

 well as observe, for a man may be a good observer and 

 not a good thinker, and a good thinker may be but a poor 

 observer. Some observers, by always applying themselves 

 to the elucidation of minute things, have apparently made 

 themselves mentally incapable of broad generalisation and 

 the understanding of great ones. 



The greatest possible caution is necessary in making 

 deductions from observed facts. So long as an observer 



