EVOLUTION AND NATURAL SELECTION 21 



about primarily by climatic conditions, but secondarily by 

 natural selection. These adaptations have been affected by 

 reason of the interrelation of insects and humming-birds to 

 flowers, and also their adjustment to change of climate. The 

 columbine blossoms in May at a time when the humming- 

 bird and hawk-moth, which participate in its polhnation, make 

 their appearance. One of the most beautiful examples of 

 adaptation found in our temperate chmate is the Uttle orange- 

 tip butterfly with its associated white-flowered Arabis. The 

 butterfly pollinates this flower while it feeds upon the sweets, 

 and in the insect's visits it alights on the blossoms, closing 

 its wings, and becomes almost invisible to human eyes. The 

 greenish white markings on the underwings cause the butter- 

 fly's body to assimilate perfectly with the flower head, thus 

 affording it protection from predaceous fees. Further on in 

 the text I have treated this subject at greater length, and there 

 wiU be found a colored illustration of the butterfly as it 

 appeared on the flower. (See plate facing page 84.) 



Other flowers, such as I have shown in the colored illustra- 

 tions, blossom at other periods throughout the various seasons, 

 each being adapted to its respective insect visitors upon 

 which it is dependent for fertilization. Each of these flower 

 forms presents a different type of color pattern and hue, 

 often disposed to lure unconsciously its winged insects. This 

 periodicity of the appearance of flowers has received considerable 

 attention by naturalists, and they have shown in many instances 

 the marvellous perfection of adaptation of structure having 

 particular value to both insect and flower.^ 



Not only do we find many flowers luring visiting insects 

 serving for fertilization, but we have instances of plants trapping 

 insects and absorbing nutrition from them. These plants are 

 provided with special structures which catch their insect prey. 

 A familiar example is the common sundew, Drosera, which 

 ensnares insects by means of sensitive tentacles disposed on 

 the surface of the leaves. Similarly, the pitcher plant of our 

 Northern States is provided with tubular leaves which contain 

 water, to which insects are attracted and in which they drown. 

 A digestive ferment is secreted which acts upon the bodies of the 



' See under Plant Adaptations, p. 31. 



