1480 THE STORY OF THE UNIVERSE 



beings remote in the scale of nature. This is like- 

 wise sometimes the case with those which may be 

 strictly said to struggle with each other for existence, 

 as in the case of locusts and grassfeeding quadrupeds. 

 But the struggle will almost invariably be most se- 

 vere between the individuals of the same species, 

 for they frequent the same districts, require the same 

 food, and are exposed to the same dangers. In the 

 case of varieties of the same species, the struggle will 

 generally be almost equally severe, and we some- 

 times see the contest soon decided: for instance, if 

 several varieties of wheat be sown together, and the 

 mixed seed be resown, some of the varieties which 

 best suit the soil or climate, or are naturally the most 

 fertile, will beat the others and so yield more seed, 

 and will consequently in a few years supplant the 

 other varieties. To keep up a mixed stock of even 

 such extremely close varieties as the variously 

 colored sweet peas, they must be each year harvested 

 separately, and the seed then mixed in due propor- 

 tion, otherwise the weaker kinds will steadily de- 

 crease in number and disappear. So again with the 

 varieties of sheep; it has been asserted that certain 

 mountain varieties will starve out other mountain 

 varieties, so that they can not be kept together. The 

 same result has followed from keeping together dif- 

 ferent varieties of the medicinal leech. It may even 

 be doubted whether the varieties of any of our do- 

 mestic plants or animals have so exactly the same 

 strength, habits, and constitution, that the original 

 proportions of a mixed stock (crossing being pre- 

 vented) could be kept up for half a dozen genera- 



