CHANGING ANIMALS 13 



vironment is called preadaptation or prospective adaptation (Simpson, 

 1953). 



Sometimes the change in question may be useful in the original environ- 

 ment and yet be of such a nature that it preadapts its possessor for life 

 in another environment. Crossopterygian fishes (pp. 157-162), for ex- 

 ample, had a fin adapted for locomotion in the water, yet the fin had 

 within it a skeleton which could be made over to form a Hmb for locomo- 

 tion on land. Thus we say that the skeleton of the crossopterygian fin 

 (Fig. 8.18, p. 161 ) was preadaptive for life on land. 



Apparently preadaptation has played an important role in progressive 

 evolution, by which we mean the production of radical changes in animals, 

 as contrasted with the perfecting of adaptation to the environment in 

 which the animal is already living. To be sure, the perfecting of the adap- 

 tation of a species to its environment is in a sense progressive. Yet such a 

 process, useful as it is, does not usually lead to radical change in structure. 

 It is one thing for a fish to become more and more perfectly adapted for 

 life in the water, quite another thing for it to climb out of the water and 

 enter the new environment of air. Fishes (i.e., the Crossopterygii) pos- 

 sessing structures preadapted for life on land could make the change; 

 other fishes could not. 



Meeting Environmental Changes 



Consideration of preadaptation has introduced a new factor into our 

 discussion, that of change in the environment. In earlier pages of this 

 chapter we noted that environmental change has occurred repeatedly 

 throughout the history of the earth. How do species of animals meet such 

 changes? 



In the first place, species frequently meet changed conditions by suc- 

 cumbing to them — by becoming extinct. Such extinction may be world- 

 wide, or it may involve only certain regions of the earth. Thus, for exam- 

 ple, at the close of the Mesozoic era the dinosaurs became extinct 

 throughout the earth. On the other hand, in much later times the camels, 

 formerly inhabitants of North America, became extinct on that continent 

 while remaining existent in Asia, Africa, and South America (llama) 

 (Fig. 12.5, p. 271). 



Another manner in which animals may meet change in the environment 

 is by being sufficiently adaptable or versatile so that they can live under a 

 great variety of conditions. While this seems not to have been a very com- 

 mon solution of the problem, various examples come to mind. The rats 



