830 THE STRUCTURE OF FLOWERS. 



less useful clianges or variations, only to be discarded at 

 eacli generation for the one form that was wanted. Thirdly, 

 as a great number of flo^yers would be visited, both on one 

 plant and on many surrounding individuals in the neigh- 

 bourhood, great numbers might bear offspring advancing 

 more or less in the same direction ; and there would be no 

 fear of extermination, even if some happened tb be crossed 

 by the parent form. Indeed, the varying offspring would 

 largely supersede the parent form in number altogether, if 

 they sprang up at one place without emigration. If we 

 supply the additional aid of isolation, many otter influences 

 would be brought to bear upon them, and they would be free 

 to vary without any interference from the parent stock. 



Mr. Darwin has abundantly shown that when a plant is 

 crossed, and its seedlings strugi^le in a confined place with 

 those derived from flowers which, have not been crossed but 

 artificially self-fertilised, they generally succeed in mastering 

 the latter ; so that if there be any struggle with the seedlings 

 of a self-fertilised parent, such a struggle for life is mainly 

 during the early period of growth, before any varietal or 

 specific characters of the flowers have put in an appearance 

 at all. For it is only in the youthful stages that the greatest 

 contest is maintained ; and the result depends largely upon 

 constitutional, and not at all upon specific, that is morpho- 

 logical characters, mostly taken from the flowers. Now, 

 Mr. Darwin has shown that such constitutional vigour does 

 very generally accompany at least the first few years of 

 crossing. So that we have a vera causa of the success 

 of such newly crossed plants in the preliminary struggle 

 for life. It need hardly be remarked that if insects thus 

 start a new variety, they are crossing the flowers at the 

 same time. 



It is true that the stimulus of crossing does not last for 



