CHANGING ANIMALS 9 



lived, changing features of the landscape. Terrestrial environments, lack- 

 ing the stabilizing influence exerted by a watery medium, are most variable 

 of all. As a rule terrestrial animals face greater fluctuations in temperature, 

 humidity, and other environmental factors than do aquatic animals. 



What have such changes in the external world to do with changes in ani- 

 mals themselves? Simply this: if a species of animal is to succeed it must 

 at all times be adapted to its environment. If the environment changes, 

 as we have seen that it does repeatedly, the species must either adjust to 

 that environmental change or die. The geologic record is fufl of examples 

 of animals that could not adjust to changed conditions and hence became 

 extinct. 



We may well note at this point that change in one species will inevitably 

 lead to changes in other species. Change in the organic environment of an 

 animal may be at least as important as change in its physical environment. 

 An animal may, for example, become adapted to a diet consisting of a 

 certain plant, as the koala, the marsupial "teddy bear," is dependent upon 

 a diet of eucalyptus leaves. If the climate changes so that the plant can no 

 longer exist in the region, the animal must either change its food habits 

 or become extinct in that region. If it becomes extinct, that fact will aflfect 

 the fate of flesh-eating animals (predators) which had been dependent 

 upon the plant eater as part of their food supply. And changes in num- 

 bers of predators will aff'ect the numbers of other species of plant eaters 

 preyed upon. So one change sets ofl" a whole series of other changes, the 

 effects expanding like the ripples started by one stone dropped in a quiet 

 pond. 



Thus we see that changes in the physical environment and changes in 

 the organic environment make change in a species inevitable if it is to con- 

 tinue inhabiting this changing world. As we have intimated, these changes 

 to be efi'ective must adapt the species to live under the conditions in which 

 it finds itself, or, alternatively, to live under some conditions available to 

 it. by migration perhaps. In the following chapters we shall see examples 

 of such adaptations in modern animals. We shall also note that despite 

 changes necessitated by the requirements of life under particular condi- 

 tions, species retain basic similarities of structure which can best be ex- 

 plained as indications of their ancestry. Both the adaptive changes and 

 the basic similarities are important to the study of evolution. 



Changing Genes 



We have noted that the geologic record gives testimony that animals do 

 change, and that the demands of living in a changing world insure that ani- 



