128 THE DARWINIAN HYPOTHESIS 



more nor less numerous than they were ; and yet we 

 know that the annual produce of every pair is from one 

 to perhaps a million young, so that it is mathematically 

 certain that, on the average, as many are killed by natural 

 causes as are born every year, and those only escape 

 which happen to be a little better fitted to resist destruc- 

 tion than those which die. The individuals of a species 

 are like the crew of a foundered ship, and none but good 

 swimmers have a chance of reaching the land. 



Such being unquestionably the necessary conditions 

 under which living creatures exist, Mr. Darwin discovers 

 in them the instrument of natural selection. Suppose 

 that in the midst of this incessant competition some 

 individuals of a species (A) present accidental variations 

 which happen to fit them a little better than their fellows 

 for the struggle in which they are engaged, then the 

 chances are in favour, not only of these individuals being 

 better nourished than the others, but of their predomi- 

 nating over their fellows in other ways, and of having a 

 better chance of leaving offspring, which will of course 

 tend to reproduce the peculiarities of their parents. Their 

 offspring will, by a parity of reasoning, tend to predominate 

 over their contemporaries, and there being (suppose) no 

 room for more than one species such as A, the weaker 

 variety will eventually be destroyed by the new destructive 

 influence which is thrown into the scale, and the stronger 

 will take its place. Surrounding conditions remaining 

 unchanged, the new variety (which we may call B) 

 supposed, for argument's sake, to be the best adapted for 

 these conditions which can be got out of the original stock 

 will remain unchanged, all accidental deviations from 

 the type becoming at once extinguished, as less fit for their 

 post than B itself. The tendency of B to persist will grow 

 with its persistence through successive generations, and it 

 will acquire all the characters of a new species. 



But, on the other hand, if the conditions of life change 

 in any degree, however slight, B may no longer be that 

 form which is best adapted to withstand their destructive, 

 and profit by their sustaining, influence ; in which case if 

 it should give rise to a more competent variety (C), this 

 will take its place and become a new species ; and thus, 

 by natural selection, the species B and C will be successively 

 derived from A. 



That this most ingenious hypothesis enables us to give 



