176 FKAGMENTS OF SCIENCE. 



of the number that can be supported; hence at some 

 period or other of their lives there must be a struggle 

 for existence. And what is the infallible result? If 

 one organism were a perfect copy of the other in regard 

 to strength, skill, and agility, external conditions would 

 decide. But this is not the case. Here we have the 

 fact of variety offering itself to nature, as in the former 

 instance it offered itself to man; and those varieties 

 which are least competent to cope with surrounding 

 conditions will infallibly give way to those that are 

 most competent. To use a familiar proverb, the weak- 

 est goes to the wall. But the triumphant fraction 

 again breeds to over-production, transmitting the quali- 

 ties which secured its maintenance, but transmitting 

 them in different degrees. The struggle for food again 

 supervenes, and those to whom the favourable quality 

 has been transmitted in excess, will triumph as before. 



It is easy to see that we have here the addition of 

 increments favourable to the individual, still more rigor- 

 ously carried out than in the case of domestication; 

 for not only are unfavourable specimens not selected 

 by nature, but they are destroyed. This is what Mr. 

 Darwin calls ' Natural Selection,' which acts by the 

 preservation and accumulation of small inherited modi- 

 fications, each profitable to the preserved being. With 

 this idea he interpenetrates and leavens the vast store 

 of facts that he and others have collected. We can- 

 not, without shutting our eyes through fear or pre- 

 judice, fail to see that Darwin is here dealing, not with 

 imaginary, but with true causes; nor can we fail to 

 discern what vast modifications may be produced by 

 natural selection in periods sufficiently long. Each 

 individual increment may resemble what mathema- 

 ticians call a ' differential ' (a quantity indefinitely 

 small); but definite and great changes may obviously 



