SO THE PERPETUATION OF LIVING BEINGS, 



having passed any length of time in tropical countries. 

 You may also alter the development of the muscles 

 very much, by dint of training ; all the world knows 

 that exercise has a great effect in this way ; we always 

 expect to find the arm of a blacksmith hard and wiry, 

 and possessing a large development of the brachial 

 muscles. No doubt, training, which is one of the forms 

 of external conditions, converts what are originally 

 only instructions, teachings, into habits, or, in other 

 words, into organizations, to a great extent ; but this 

 second cause of variation cannot be considered to be 

 by any means a large one. The third cause that I 

 have to mention, however, is a very extensive one. It 

 is one that, for want of a better name, has been called 

 " spontaneous variation ; " which means that when we 

 do not know anything about the cause of phenomena, 

 we call it spontaneous. In the orderly chain of causes 

 and effects in this world, there are very few things of 

 which it can be said with truth that they are spon- 

 taneous. Certainly not in these physical matters, — in 

 these there is nothing of the kind, — everything depends 

 on previous conditions. But when we cannot trace the 

 cause of phenomena, we call them spontaneous. 



Of these variations, multitudinous as they are, but 

 little is known with perfect accuracy. I will mention 

 to you some two or three cases, because they are very 

 remarkable in themselves, and also because I shall 

 want to use them afterwards. Reaumur, a famous 

 French naturalist, a great many years ago, in the essay 

 which he wrote upon the art of hatching chickens, — 

 which was indeed a very curious essay, — had occasion 

 to speak of variations and monstrosities. One very re- 

 markable case had come under his notice of a variation 



