Arrangements for Securing Union of Sexes 317 



117. Dimorphous flowers, 

 enlarged, of Partridge Berry, 

 further explained in the text. 

 (Reduced from Gray's Struc- 

 tural Botany). 



on the principle that although cross fertilization is better than 

 close, a close fertilization is better than none. And for this 

 principle there is much other evidence. In all of these cases, 

 however, at least an occasional cross fertilization must be ef- 

 fected; and there is good reason to be- 

 lieve that while many kinds of plants 

 can endure close fertilization for a con- 

 siderable time, they must have an oc- 

 casional cross in order 'to retain their 

 full vigor. 



But we must turn for a moment to 

 view cross pollination from the side of 

 the insect. Our discussion thus far may 

 have seemed to imply that insects Fl 

 exist in certain sizes, forms, and habits, 

 fixed by other considerations, and that 

 the adaptations between them and 



flowers have been wholly effected by modifications of the flow- 

 ers. This, however, is not correct, for there is every evidence 

 that in the course of evolution, insects have become adapted 

 to flowers as well as flowers to insects, as indeed we might expect 

 from the fact that while it is an advantage for flowers to have 

 their pollen carried by insects, it is an advantage to insects to be 

 able to obtain their food from the flowers. There are, of course, 

 many kinds of insects which never visit flowers at all; and it is 

 only the kinds which are nectarivorous, so to speak, that plants 

 have been able to provide an attraction for, and only these kinds 

 show adaptations to flowers. 



For the success of cross pollination of flowers by insects, it is 

 obviously essential that the insects shall habitually visit plant 

 after plant of the same kind, rather than first one kind of plant 

 then another, which happen to blossom together. For no result 

 follows a cross pollination between different kinds. Observation 

 always shows that in fact insects do as a rule visit the same kinds 



