264 THE WONDER OF LIFE 



constantly discovering new ones to-day. Every move on 

 Nature's chessboard has consequences which may have a 

 very long-lasting influence on the game. We know that the 

 housefly puts an appreciable drag on the wheel of civiliza- 

 tion, that squirrels affect the harvest, that wagtails have 

 to do with the success of sheep-farming, and that cats may 

 play a not unimportant role in determining the welfare of 

 India. 



As a corollary to Darwin's central conception came 

 Pasteur's — the idea of the controUabihty of life. Silk- 

 worm disease and Phylloxera among the vines are not dis- 

 pensations of Providence to be submitted to, they are 

 handicaps to be got rid of. Olive pests ia Italy and Vole 

 plagues in Thessaly do not arise without good reason, 

 and it is within our powers to alter these reasons. Tollitur 

 causa, ablatus effectus. 



The Balaijce of Nature 



This phrase may serve to indicate the broadest kind of 

 inter-relation, where two sets of living creatures, having 

 evolved together, are dependent on one another, and on 

 the persistence of an approximate equilibrium between 

 them. It is possible to construct a closed-off aquarium 

 in which the plants and animals balance one another per- 

 fectly for a period varying with the degree of uniformity 

 in external conditions, and the carefulness of adjustment 

 between the diverse constituents of the population. The 

 oxygen required by the animals is produced in sunhght 

 by the green plants, and the carbonic acid gas produced 

 by the animals is utilized by the plants. The closed-off 

 microcosm usually comes to an end by an over-production 



