VARIATION. 71 



is, to try to adapt themselves to their new cir- 

 cumstances. Sometimes where nutritious food 

 is less easily obtained the animal deteriorates, 

 but is at the same time better adapted to its 

 surroundings ; again, where nutritious foods be- 

 come more abundant a corresponding change 

 for the better occurs. The Shetland ponies 

 well illustrate this. " They are perfectly adapted 

 to their bleak and barren habitat, and this 

 adaptation is certainly due to a deterioration 

 from a larger and more active and more ele- 

 gantly formed beast. The coarse bone, spe- 

 cially notable in the disproportionately large 

 and ill-formed head, shows this, and they have 

 developed a thick suit of wool in addition to 

 the natural coat to protect them against the ex- 

 treme cold, which comes on in the autumn and 

 is early shed as spring deepens into summer. 

 Imported into warmer climes these ponies in a 

 few generations show a tendency to increase.in 

 size and lose the auxiliary coat of woolly hair. 

 Wherever an animal or a genus finds it possi- 

 ble to adapt itself to its changed conditions it 

 does so and survives, but many become extinct. 

 We have in the geological record some remark- 

 able examples of efforts on the part of animals 

 to adapt themselves to such changes. Thus a 

 mollusk family which was once very abundant, 

 but which is now very rare, has left a wonder- 

 ful record of its extraordinary efforts to main- 



