128 THE CAUSATION OF DISEASE. 



It is obvious that such a proposition can admit of no experi- 

 mental proof. How is it possible to subject any two brothers 

 or sisters to exactly the same E from the earliest period 

 of their existence, the E, it must be remembered extending back 

 beyond the period of embryonic life to the actual coming into 

 being of germ and sperm? But, although our proposition admits 

 of no absolute experimental proof, neither does the first law of. 

 motion. The two, indeed, allow of the same kind of proof — 

 namely, that termed by logicians the " method of residues." 

 The first law of motion runs thus : "A body in motion will con- 

 tinue to move with the same velocity in a straight line for ever, 

 if no other force than that by which it is impelled act upon it." 

 But the conditions of the "if" obtain nowhere. Nowhere 

 is it possible to isolate a body from the action of other forces ; 

 the tiniest fleck acts upon, and is acted upon by, every other 

 particle of matter in the universe, however remote it be. He 

 who denies this must be prepared to confute the law of universal 

 gravitation. It is found, however, that the nearer the conditions 

 of the " if " are approached, the closer does the result accord 

 with the terms of the proposition. In our own world, friction 

 and atmospheric resistance are ever present to counteract the 

 tendency of the first law of motion, but such outside resisting 

 forces are reduced to a minimum in the case of the heavenly 

 bodies, which, suffering practically no resistance, maintain a 

 more or less uniform rate of movement ; nevertheless, the 

 motion is not in a uniform straight line, for gravity still acts, 

 causing the movements in curves, and maintaining eacli system 

 in "moving equilibrium."* 



The same line of argument applies to heredity : the nearer 

 the requirements of the case are complied with, the closer do 

 we find the result according with the assertion. In both cases 

 we have to deal with an E which represents the conditions of 

 the " if ; " in the one, we require a completely negative E, and 

 in the other an E which shall be alike for all the children born 

 of the same parents, and it is, as I have said, manifestly im- 

 possible to obtain either of these requisites. 



* The truth of the principle, however, is placed beyond the possibility of 

 confutation by the fact that it forms a correct basis for astronomical calcu- 

 lation. 



