76 
STRUGGLE FOR EXISTENCE. 
Chap. III. 
coloured sweet-peas^ they must be each year harvested 
separately, and the seed then mixed in due propor¬ 
tion, otherwise the weaker kinds will steadily decrease 
in numbers 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 whe¬ 
ther the varieties of any one of our domestic plants 
or animals have so exactly the same strength, habits, 
and constitution, that the original proportions of a 
mixed stock could be kept up for half-a-dozen genera¬ 
tions, if they w^ere allowed to struggle together, like 
beings in a state of natmu, and if the seed or young 
were not annually sorted. 
As species of the same genus have usually, though 
by no means invariably, some similarity in habits and 
constitution, and always in structure, the struggle will 
generally be more severe between species of the same 
genus, when they come into competition with each other, 
than between 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 an¬ 
other species. The recent increase 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. One species of charlock will supplant another, 
and so in other cases. We can dimly see why the com¬ 
petition should be most severe between allied forms, 
which fill nearly the same place in the economy of nature; 
