XXII.] WHERE MOST RAPID CHANGES OCCUR. 357 



out ; " for if they do pass away new varieties must 

 take their places, or the cultivated types of the 

 species would cease to exist. Or, to state the propo- 

 sition differently, if varieties run out, the species can 

 be rescued from oblivion only by new forms ; but 

 inasmuch as all valuable cultivated plants tend con- 

 stantly to increase in extent of cultivation, it follows 

 either that they do not run out, or that new varieties 

 are better than the old and drive them out. And yet 

 there are persons who hold tenaciously to both 

 dogmas, — that varieties run out and that novelties do 

 not pay, — without seeing that the logical result of 

 such opinion is to erase the cultivated flora from the 

 face of the earth. Now, it is true that the varieties 

 of any plant are, as a whole, constantly changing, 

 as one may prove by comparing the catalogues and 

 manuals of a generation ago with those of to-day. 

 These changes are most rapid in plants of shortest 

 duration, or those in which there has been the 

 greatest number of generations, showing that the 

 greater the opportunity for renewal of stock the 

 greater is the variation and number of recorded 

 varieties. Thus the apples of to-day are as much 

 like those of a century ago as the strawberries of 

 to-day are like those of ten years ago ; and there 

 is about the same number of generations in the one 

 ease as in the other. This means, as I said before, 

 that the rate of change in named varieties is in pro- 

 portion to the length of life or profitable duration 

 of the species. This at once raises a strong pre- 

 sumption that varieties do not wear out from mere 

 age, but that they pass out by variation in the pro- 

 cess of reproduction ; and as varieties of standard 



