48 HEREDITY 



able. We find, on the contrary, as has been illus- 

 trated in the previous chapter, that these facts, 

 though admittedly incalculable for the individual, 

 are yet capable of mathematical expression, often of 

 the simplest, when we consider large numbers of 

 individuals. Similarly the physicist, be he a Kelvin, 

 is unable precisely to predict the size and course of 

 all the fragments produced by an explosion. Yet he 

 knows that the laws of motion and gravitation and 

 chemical combination and gaseous pressure are ob- 

 served, and his knowledge is not affected by the 

 circumstance that his limited powers do not suffice 

 for precise prediction in such a case ; nor would he 

 regard with anything but sympathy the writer who 

 should say, "Well, I am content to conclude that, 

 since no one can predict the course and weight of 

 even one of these fragments, either there are no laws 

 in the matter, or else they are wholly unknown." 

 This is precisely what is said by the critics in relation 

 to the study of such subjects as variation; and the 

 criticism is as worthless in one case as in the other. 

 When variation is studied on a sufficiently ample 

 scale — in accordance with the general principles of a 

 'posteriori reasoning, for the more numerous the 

 foundation-facts, the more secure and comprehen- 

 sive must be the generalisation erected upon them — 

 we find the facts are capable of being expressed in 

 certain general propositions, such as are commonly 

 called laws in scientific writings. 



There is a fundamental distinction between a law 

 and a cause, as the case of gravitation suffices to 

 illustrate. Nevertheless, we are surely entitled to 

 assume that the occurrence of a law, i.e. of a natural 



