1 68 DARWINISM AND THE PROBLEMS OF LIFE 



We have already recognised the significance of 

 selection in the reduction of organs. I may remind 

 the reader of the wings of the island-insects and the 

 ringers of the bird. There are snails, too, with small 

 shells right at the posterior end. They feed on earth- 

 worms, and it is possible that their malformation is due 

 to the fact that the snails with the smallest shells were 

 best able to follow their prey into their holes. There 

 may be many cases that selection can explain, but 

 as we saw, it cannot explain all the rudimentary 

 organs. 



Another principle is that of economy of nourishment. 

 Every animal, it says, has a limited amount of nourish- 

 ment in its body, and if one of its organs is to grow 

 bigger, the material for it must be taken from another 

 part of the body. If, for instance, in a certain species 

 the strongest possible support for the body is required, 

 as in swift-running animals of a certain weight, the shin- 

 bone must become thicker, as this is always the chief 

 support of the body. In that case natural selection will 

 always choose the animals with the strongest shin-bones 

 (tibiae), and possibly at the same time, indirectly, select 

 smaller calf-bones (fibulae) if, namely, the shin-bone 

 has obtained its increase in size at the expense of the 

 fibula. Now, there is certainly only a limited quantity 

 of nourishment in the bones, and it seems possible that 

 for one bone to become larger it must take the food that 

 would have gone to the other bone. In this way the 

 continuous selection of strong shin-bones might gradually 

 bring down the fibula to the dimensions it actually has 

 in the horse and the bird. 



