8 ORGANIC EVOLUTION— PHYSICAL 



infer from them with mathematical certainty that it 

 must have occurred. It is a matter of common know- 

 ledge and experience (1) that in both the animal and 

 vegetable kingdoms every individual differs somewhat 

 from every other individual, but that (2) while the 

 offspring is never an exact copy of the' parent, there is 

 yet a tendency for the peculiarities of the parent to 

 reappear in the offspring in a greater or lesser degree. 

 Whence it is clear, if some individuals of a species 

 possess a peculiarity, such as a superior keenness of sight, 

 or of hearing, or of scent, or a power of muscle, or a 

 capacity of resisting heat, or cold, or hunger, or thirst, 

 or disease, &c., in a greater degree tban that possessed 

 by the other members of the species, under conditions 

 which render this peculiarity of importance in the 

 struggle for existence, that the individuals which 

 possess the peculiarity in the greater degree will be 

 at an advantage, and will, on the whole, survive in 

 greater numbers, and, consequently, other things equal, 

 have a more numerous offspring than those which 

 possess the peculiarity in the lesser degree. And 

 further, since the offspring, while inheriting their 

 parents' peculiarities, tend to vary somewhat from them, 

 there will, in the next generation, probably be some 

 who have the peculiarity in a greater degree than their 

 parents, and others who have it in a lesser degree, when, 

 if the conditions remain the same, there will be such a 

 new survival of the fit, and such a fresh elimination of 

 the unfit, as will leave the survivors in the third 

 generation with the peculiarity developed, in however 

 slight a degree, more than it was developed in their 

 grandparents ; and it follows that this process, repeat- 

 ing itself through innumerable generations, and during 

 long epochs of time, will at length develop the peculiarity 

 to a point beyond which any further increase is no 

 longer useful, or to a point beyond which it is so little 



