784 ANIMALS AND THEIR ENVIRONMENT 



353. Structural Adaptations 



Animals become adapted tor a particular mode of life in a par- 

 ticular environment by specializations of structure, function, color, 

 chemical composition or behavior. Structural adaptations are, perhaps, 

 the most easily recognized; changes in the size, shape, relative propor- 

 tion and so on of the bones and muscles of the body which adapt for 

 running, jumping, climbing, gliding, flying, burrowing or swimming 

 are, in general, readily evident. The adaptive nature of some other 

 structural modifications only becomes clear when an animal is studied 

 in its environment. 



In many animals, the specialized adaptation to a certain way of life 

 now evident is simply the latest stage in a series of adaptations. For 

 example, both man and the baboon, whose immediate ancestors were 

 tree-dwellers, have returned to the ground and have become readapted 

 for walking rather than climbing trees. The process of readaptation may 

 be quite complicated. The contemporary Australian tree-climbing kan- 

 garoos are the descendants of an original ground-dwelling marsupial. 

 From these ground-dwellers evolved forms which, in the course of adap- 

 tive radiation, took to the trees and developed limbs adapted to tree 

 climbing (or perhaps the sequence of events was the reverse— first the 

 evolution of specialized limbs and then the adoption of an arboreal 

 life). Some of these tree-dwellers eventually left the trees and became 

 readapted for ground life, accumulating, by mutation and selection, 

 genes for hind legs which were longer, stronger, and adapted for leap- 

 ing. Some of these readapted ground-dwellers then returned to the trees 

 in the course of further evolution, but their legs were so highly special- 

 ized for leaping that they could not be used for grasping a tree trunk. 

 In consequence, the present-day tree kangaroos must climb like bears, 

 by bracing their feet against the tree trunk. A comparison of the feet of 

 the existing Australian marsupials reveals all the stages in this compli- 

 cated, shifting process of adaptation. 



354. Physiologic and Chemical Adaptations 



Since one of the major struggles among organisms stems from the 

 competition for food, any mutation which enables an animal to utilize 

 a new type of food will be extremely advantageous. This might in- 

 volve the evolution of a new digestive enzyme or of a new energy-liberat- 

 ing enzyme system. The evolution of a new enzyme system enables the 

 sulfur bacteria to obtain biologically useful energy from hydrogen sul- 

 fide, a substance which is poisonous to almost all other organisms. The 

 evolution of a special enzyme for reducing disulfide bridges gives the 

 clothes moth its unique ability to digest wool, the protein molecules of 

 which are held together by such disulfide bridges. 



A mutation that decreases the growing season of a plant or the 

 total length of time required for an insect or other animal to complete 

 development will enable it to survive farther from the equator, thus 

 opening up new areas of living space and new sources of food for the 



