230 



EVOLUTION AND GENETICS 



mediate between the insectivores and bats, and by the entire order 

 Chiroptera, the bats. 



The Pterosaurs had wings formed of sldn folds stretched between 

 the hind-limb and one greatly elongated digit of each fore-limb 

 (Fig. 139). In Galeopitheciis the membrane stretches from limb 

 to limb, and to the tail, and in the bats a similar condition pre- 

 vails, but the elongation of several digits of the fore-hmb extends 

 the membrane into a wing-like form. 



Adaptation to the Organic Environment. The Web of Life. 

 The relationship of organisms with each other is infinitely com- 

 plex. All organisms depend 

 directly or indirectly upon 

 photosynthesis for food. 

 The green plants carry on 

 the process for themselves, 

 and some animals feed upon 

 the green plants, but other 

 animals secure their food by 

 eating animals. Some or- 

 ganisms depend upon others 

 for protection or conceal- 

 ment, for tempering the 

 intensity of hght, for shelter 

 from the weather and for 

 many other things. The 

 resulting complexity has 

 given rise to the conception 

 of the web of life, in which 

 organic associations are 

 likened to a woven fabric. 

 The disturbing of one 

 thread affects the relation- 

 ships of others. The more 

 intimate the association with the source of disturbance, the greater 

 the change, but step by step the effect may be transmitted 

 throughout the whole fabric. 



Darwin called attention to a striking illustration of such inter- 

 action. He noted that red clover was fertilized by bumble bees, 

 and that protected flower heads did not produce seed. Since field 

 mice destroy the nests of bumble bees, they are a check on the 



Fig. 137. 



—The flying dragon, Draco volans, 

 a lizard. (From Lull.) 



