DURATION OF RACES. 345 



meaning that each individual attacked does not die 

 of old age, but of manifest disease it may he asked in 

 return, what individual man ever dies of old age in any 

 other sense than of a similar inability to resist inva- 

 sions which in earlier years would have produced no 

 noticeable effect ? Aged people die of a slight cold 

 or a slight accident, but the inevitable weakness that 

 attends old age is what makes these slight attacks fatal. 



Finally, there is a philosophical argument which , 

 tells strongly for some limitation of the duration of 

 noil-sexually-propagated forms, one that probably 

 Knight never thought of, but which we should not 

 have expected recent writers to overlook. When Mr. 

 Darwin announced the principle that cross-fertilization 

 between the individuals of a species is the plan of 

 Nature, and is practically so universal that it fairly 

 sustains his inference that no hermaphrodite species 

 continually self-fertilized would continue to exist, he 

 made it clear to all who apprehend and receive the 

 principle that a series of plants propagated by ifuds 

 only must have weaker hold of life than a series re- 

 produced by seed. For the former is the closest pos- 

 sible kind of close breeding. Upon this ground such 

 varieties may be expected ultimately to die out ; but 

 " the mills of the gods grind so exceeding slow " that 

 we cannot say that any particular grist has been actu- 

 ally ground out under human observation. 



If it be asked how the asserted principle is proved 

 or made probable, we can here merely say that the 

 proof is wholly inferential. But the inference is 

 drawn from such a vast array of facts that it is well- 

 nigh irresistible. It is the legitimate explanation of 



