CHAP. III.] STRUGGLE FOE EXISTENCE. 55 



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-coloured sweet peas, they must be each year 

 harvested separately, and the seed then mixed in due proportion, 

 otherwise the weaker kinds will steadily decrease 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 cannot be kept together. The 

 same result has followed from keeping together different varieties 

 of the medicinal leech. It may even be doubted whether the 

 varieties of any of our domestic plants or animals have so exactly 

 the same strength, habits, and constitution, that the original 

 proportions of a mixed stock (crossing being prevented) could be 

 kept up for half-a-dozen generations, if they were allowed to 

 struggle together, in the same manner as beings in a state of 

 nature, and if the seed or young were not annually preserved in 

 due proportion. 



Struggle for Life most severe between Individuals and Varieties of 

 the same Species. 



As the species of the same genus usually have, though by no 

 means invariably, much similarity in habits and constitution, and 

 always in structure, the struggle will generally be more severe 

 between them, if they come into competition with each other, than 

 between the species of distinct genera. We see this in the recent 

 extension over parts of the United States of one species of swallow 

 having caused the decrease of another species. The recent in- 

 crease of the missel-thrush in parts of Scotland has caused the 

 decrease of the song-thrush. How frequently we hear of one 

 species of rat taking the place of another species under the most 

 different climates ! In Russia the small Asiatic cockroach has 

 everywhere driven before it its great congener. In Australia the 

 imported hive-bee is rapidly exterminating the small, stingless 

 native bee. One species of charlock has been known to supplant 

 another species ; and so in other cases. We can dimly see why 

 the competition should be most severe between allied forms, which 

 fill nearly the same place in the economy of nature ; but probably 

 in no one case could we precisely say why one species has been 

 victorious over another in the great battle of life. 



A corollary of the highest importance may be deduced from the 

 foregoing remarks, namely, that the structure of every organic 

 being is related, in the most essential yet often hidden manner, tc 

 that of all the other organic beings, with which it comes into com- 

 petition for food or residence, or from which it has to escape, or 



