602 Mutations 



On the other haud occasional specimens with a 

 single variegated leaf, or with some few of them, 

 are actually met with, and if attention is once 

 drawn to this question, perhaps a dozen or so 

 instances might be brought together in a sum- 

 mer. But they never seem to be capable of 

 further evolution, or of reproducing themselves 

 sufficiently and of repeating their peculiarity in 

 their progeny. They make their appearance, 

 are seen during a season, and then disappear. 

 Even this slight incompleteness of some spots 

 on one or two leaves may be enough to be their 

 doom. 



It is a common belief that new varieties owe 

 their origin to the direct action of external 

 conditions and moreover it is often assumed that 

 similar deviations must have similar causes, and 

 that these causes may act repeatedly in the same 

 species, or in allied, or even systematically dis- 

 tant, genera. No doubt in the end all things 

 must have their causes, and the same causes 

 will lead under the same circumstances to the 

 same results. But we are not justified in deduc- 

 ing a direct relation between the external con- 

 ditions and the internal changes of plants. 

 These relations may be of so remote a nature 

 that they cannot as yet be guessed at. There- 

 fore only direct experience may be our guide. 



Summing up the result of our facts and dis- 



