NATURAL SELECTION 133 



of the shot from the gun of B, who is shooting at a rabbit. We 

 may discover A's motive for walking in the wood ; in the case 

 of B the desire to shoot rabbits is the obvious motive. But the 

 crossing of the two men's lines of action at that particular moment 

 is a coincidence of which we cannot attempt an explanation. No 

 doubt, had we full knowledge, such a thing would be as calcul- 

 able as the transit of Venus. But, though this is undeniable, yet, 

 in the case imagined, as in others of everyday occurrence, the 

 intersection of the lines along which two forces act, is due to 

 chance, unless we assume a co-ordinating principle which times 

 and regulates every collision between force and force. Certainly 

 Darwin did not make this assumption. But by recognising coin- 

 cidence, he did not banish purpose from the universe and set up 

 chance in its place. His theory of evolution requires an inter- 

 play of forces unrestricted by any central regulation. But, after 

 all, this is but a limited field for chance : the forces themselves 

 are matter of chance only in the sense that we are ignorant of 

 their working. And even the limited field which he left for the 

 play of coincidence tends, as I shall show, to become further 

 narrowed as evolution proceeds. 



I will now try to define clearly at what points chance in 

 either sense enters into the Darwinian theory of evolution. 



(1) Variations are in all cases due to chance, in so far as 

 we cannot trace a particular one to its cause the secret lies 

 hidden in the germ-plasm nor predict what variation, if any, 

 will arise in a particular case. 



(2) The sexual unions which bring about new combinations 

 and so give rise to variations are largely due to coincidence. 

 If favourable variations arise in a few members of a numerous 

 species, the help of lucky coincidence will, as a rule, be required 

 to bring it about that any of these similarly varying individuals 

 shall pair together. If, however, their numbers are great, not 

 much luck will be necessary. 



(3) When an adaptation is effected, it is due to mere 

 coincidence. 



Of these three points, the third presents the greatest diffi- 

 culty. How are we to account for the correspondence between 



