136 DARWINISM AND THE PROBLEMS OF LIFE 



of the period. They were never faced with the 

 alternative of changing or dying. If this alternative 

 had been forced on the generations very gradually, 

 and always in proportion to the possible latitude of 

 variation, we may confidently assume that they would 

 not have failed to meet it because of some necessity for 

 remaining unchanged. 



We may also suppose that in all species of animals 

 any change is possible if there is time enough for it. 

 We are bound to admit this if our conception of the 

 organic world is a scientific one. The tenth and eleventh 

 chapters will tell us more of this. We will only say 

 for the present that every particle of living matter has 

 the tendency to vary. Every organ, every part of an 

 animal, may change ; and this change will in certain 

 circumstances become the basis for selection. How 

 much the structural plan of an animal may be trans- 

 formed is seen very well in the intestinal worms, some 

 of which have lost the alimentary canal altogether, and 

 take their food through the skin. The nearest character 

 will, of course, be used for change. And with what 

 enormously different means it may be effected is well 

 seen in the wings of animals. In bats and birds the 

 fore-limbs have been converted into organs of movement 

 in the air ; in the former the skin forms the air-beating 

 surface, in the latter the feathers. The birds come from 

 the reptiles probably leaping reptiles. The feathers 

 had already been developed, as a warm covering, from 

 the scales, and amongst the leapers those certainly had 

 the advantage which had the longest quills on the fore- 

 limbs, so as to act as parachutes in falling. This was 



