154 [MORPHOLOGICAL DEVELOPMENT. 



free access. In like maimer, flowers wliicli are distributed 

 over a plant in such ways tliat their discs open out on 

 planes of all directions and inclinations, will have no tend- 

 ency to lose their radial symmetry ; since, on the average, 

 no part of the periphery is diflerently related to insect- 

 agency from any other part. But flowers so fixed as to 

 open out sideways in tolerably- constant attitudes, ha"ve 

 their petals diflerently related to insect-agency. A bee or 

 butterfly coming to a laterally- growing flower, does not set- 

 tle on it in one way as readily as in another ; but almost of 

 necessity settles with the axis of its body inclined uj)wards 

 towards the stem of the plant. Hence, the side-petals of a 

 flower so fixed, habitually stand to the alighting insect in 

 relations difierent from those in which the upper and lower 

 petals stand ; and the upper and lower petals differ from one 

 another in their relations to it. If, then, there so arises an 

 habitual attitude of the insect towards the petals, there must 

 be some particular arrangement of the petals that will be 

 most convenient to the insect — will most facilitate its en- 

 trance into the flower. Thus we see in many cases, that a 

 long undermost petal or lip, by enabling the insect to settle 

 in such way as to bring its head opposite to the opening of 

 the tube, aids its fertilizing agency. But whatever be the spe- 

 cial modifications of the corolla which facilitate the actions of 

 the particular insects concerned, all of them will conduce to 

 bilateral symmetry ; since they will be alike for the two sides 

 but unlike for the top and bottom. And now we 



are prepared for understanding the exceptions. Flowers 

 gro^dng sideways can become thus adapted by survival of 

 the fittest, only if they are of such sizes and structures tliat 

 insect-agency can afiect them in the way described. But 

 in the plants named above, this condition is not fulfilled. A 

 Hollyhock-flower is so open, as well as so large, that its petals 

 are not in any appreciable degree differently related to the 

 insects which visit it. On the other hand, the flower of the 

 Agrimony is so small, that unless visited by insects of a 



