In the preceding chapters we have accumulated a body of facts and
arguments which will enable us now to deal with the very core of our
subject--the formation of species by means of natural selection. We have
seen how tremendous is the struggle for existence always going on in
nature owing to the great powers of increase of all organisms; we have
ascertained the fact of variability extending to every part and organ,
each of which varies simultaneously and for the most part independently;
and we have seen that this variability is both large in its amount in
proportion to the size of each part, and usually affects a considerable
proportion of the individuals in the large and dominant species. And,
lastly, we have seen how similar variations, occurring in cultivated
plants and domestic animals, are capable of being perpetuated and
accumulated by artificial selection, till they have resulted in all the
wonderful varieties of our fruits, flowers, and vegetables, our domestic
animals and household pets, many of which differ from each other far
more in external characters, habits, and instincts than do species in a
state of nature.
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