252 THE STRUCTURE OF FLOWERS. 



allow of the proper amount of nutrition being at the service 

 of the flowering process. This is so well known that I need 

 not dwell upon it now. The second is the cessation of insect 

 fertilisation. The effect of fertilisation operates in two 

 directions. On the one hand, if it be the result of inter- 

 crossing bj' insect agency, it stimulates the flowers till they 

 become thoroughly adapted to their visitors, and highly 

 differentiated in certain ways in consequence, but more 

 especially as regards the perianth and stamens ; while, in 

 many cases, some degree of degradation occurs simultaneously 

 in the pistil. Conversely, self-fertilisation and anemophily, 

 consequent ujDon the neglect of insects, are accompanied by 

 corresponding degradations in the perianth, stamens, and 

 pollen, correlated with a regained ascendancy in the powers 

 of reproduction. The limits of degradation, with an increase 

 of fertility, are seen in many cleistogamous flowers. 



In tracing the progress of degeneracy from a species with 

 large flowers to one with inconspicuous blossoms, I do not 

 mean to imply that we can actually witness the process in 

 activity • but we can see this represented, as it were, in many 

 a series of what we call species of a genus ; but which we 

 might call transitional forms of one kind. It is only because 

 we cannot trace the actual process going on that we regard 

 them morphologically as distinct species. Thus, if a verifiable 

 demonstration be unattainable, it is a "moral conviction,'* 

 not only that Geranium pratense is as much and obviously 

 adapted to insect agency as G. pusillum is to fertilise itself, 

 but that the latter species has been derived from the former 

 or from some kindred plant, through some such transitional 

 forms as G. pyrenaicum and G. molle. 



This process of degradation from insect to self-fertilising 

 conditions, not only affects the size of all parts of the flower, 

 but the entire plant. Mr. Darwin showed how the stimu- 



