378 THE CHANGING GENERATIONS 



may result; but if it has nothing that could serve as a starting point, 

 evolution cannot produce the desirable adaptation de novo. Much of the 

 lack of adaptation in organisms is also probably due to evolutionary 

 change lagging behind environmental change. 



Adaptive radiation. Among the species of a single family or order it 

 is usual to find a wide variety of ways of living and corresponding variety 

 in the types of situations occupied. The members of the squirrel family 

 {Sciuridae) are clearly related, although some are arboreal, others ter- 

 restrial, and still others burrowers in the soil. Yet they are all built on a 

 common plan that is modified in relation to the mode of life of each 

 species. As part of their basic inherited equipment, all of them have claws 

 and tails. In the tree squirrels the claws have become sharp spikes by 

 means of which the animals cling to tree trunks or run sure-footedly on 



Blue Racer Hog - nosed Snake 



Fig. 24.15.- Adaptive modification. The head of the black snake or blue racer {Coluber 

 constrictor) is that of a typical snake. The head of the hognose snake or puff adder (Heterodon 

 plat y rhinos), is modified for burrowing in the soil. (After D. Dwight Davis, Jr., courtesy 

 General Biological Supply House, Inc.) 



branches, digging the points into the bark as a linesman uses his climbing 

 tools. The tail has been modified into a graceful balancing organ, also 

 useful as a cloak. The flying squirrels show the same changes, but in 

 addition the skin of the sides of the body has become a loose flap which 

 can be tightened by extending the legs, and the tail hair grows out in a 

 horizontal plane. These modifications enable the animal to make his 

 entire body a plane for gliding, simply by extending the legs and tail. The 

 prairie dogs have become burrowers in the soil. Their claws are modified 

 into strong digging tools, and the useless tail is reduced to a mere stub. 

 None of these adaptations to different ways of life involves new structures 

 but only modifications of the ancestral equipment. The phenomenon here 

 illustrated is a nearly universal one — the spreading out of the descendants 

 of a single original stock into more numerous and diverse types of en- 

 vironment, with accompanying adaptive changes. We shall encounter 

 additional examples of adaptive radiation in our survey of the evolu- 

 tionary history of life. 



Adaptive convergence. Distantly related groups, each undergoing 

 adaptive radiation, may contain species adapted to the same type of 

 environment. The result may be a strong likeness in appearance and mode 

 of life between the species concerned — but a likeness due to analogous 



