ADAPTATIONS IN ANIMALS AND PLANTS 43 



flowers by insects, and here I have just witnessed the same 

 phenomenon exhibited by the ruby-throat. If observations 

 are carried further along in the season in July, August, and 

 September, it will be found that certain flowers, such as the 

 trumpet creeper, the spotted touch-me-not, and the cardinal 

 flowers are really "bird flowers." In these flowers there is 

 existing a reciprocal relation between the bird and flower, 

 just as we have observed existing between flowers and insects. 



The following are the characteristic features of these bird 

 flowers: they are erect in position; are either pouch-shaped 

 or cylindrical in form; are noted for their brilliancy of color, 

 generally either scarlet or yellow, and for the great amount 

 of nectar that they secrete. 



In the tropics, they reach great perfection and variety of 

 form, and in that latitude there exists coincidentally a great 

 variety of humming-birds, with various elongated bills, to visit 

 them. With us, there are fewer real bird flowers. These 

 are all visited by the single species of humming-bird, the ruby- 

 throat, that is found in summer over the whole temperate eastern 

 North America. It proceeds north in the spring, during the 

 vernal change in vegetation, and stays with us through the 

 summer, and in the fall returns to the south. The migration 

 of this species over such a vast extent of country is of particular 

 interest in the present connection, for in carrying pollen from 

 one flower to another situated widely apart, the cross-fertiliza- 

 tion influence gives an impetus and strengthening character 

 to the plants, as is well understood by students of plant breeding 

 and was especially noted by Darwin. 



The relation between the ruby-throat and the period of 

 blooming of bird flowers has received considerable attention 

 by naturalists. Robinson suggests ^ that the spring and autumn 

 migration of this bird may account for the tendency of these 

 bird flowers to form an early and late group. Sprengel showed 

 in his writings, in 1787, that he believed that the nectar of 

 flowers is secreted for the sake of insects and is protected from 

 rain in order that the insects may get it pure and unspoiled. 

 Without going into details here, he was evidently the first, 

 as Miiller believes, to "view the subject in the light of adapta- 

 ^ American Naturalist, February, 1895, p. 113. 



