go THEORETICAL BIOLOGY 



which is always discontinuous. It is interesting, however, to 

 show how in single instances the gaps become closed up by 

 the forms. 



I have already drawn attention to the gap that must 

 appear in the world-picture on account of the blind spot in our 

 retina, but which gets filled in by the coloured surfaces of 

 the environment immediately growing together. 



Still more obvious is the break in the series of our moment- 

 signs which comes with sleep : it is filled in by the pure form 

 of time. 



Motion, the common form of the direction-signs, is also, 

 in principle, quite continuous. It is here that the principle of 

 the continuity of the world plays its chief part, for in this 

 instance the gaps in the material of experience are most 

 apparent. Since all forces can be transformed into motion, 

 the principle of the continuity of motion has become a funda- 

 mental axiom of physics, and, in the form of the law of inertia, 

 for instance, has led to the discovery of consequences of the 

 highest importance. The theories of the propagation of 

 sound, light and heat are all based on it. 



The Force of Gravity. But the attempt to deal with 

 gravitation as motion has not yet succeeded, for propagation 

 of gravity is not demonstrable. What Newton discovered 

 under the apple-tree was not a definite property of bodies, 

 which we call weight (that was known long before), but the 

 invisible bond that connects all objects with the ground, 

 a bond so tense that it bent down the apples on the 

 branches, and sometimes tore them off. He saw this same 

 bond stretching between the moon and the earth, and he 

 realised that it drew up the tide towards the moon. He called 

 this force a force acting from a distance, which was able to 

 release motion without itself being motion. 



The force of reciprocal attraction of masses undeniably 

 exerted through space and, unlike all other natural forces, 



