| → of the species of 1859 1860 1861 1866 1869 |
| species vary in 1872 |
|
| → rare species with restricted ranges. 1859 1860 1861 |
| do the rare and restricted species. 1866 1869 1872 |
|
| → little fan of 1859 1860 1861 |
| branching and 1866 1869 1872 |
|
| → have formed 1859 1860 1861 1866 1869 |
| form it into 1872 |
|
| → generations; but it would have been better if each had represented ten thousand 1859 1860 1861 1866 |
| generations, or ten thousand. 1869 |
| or more 1872 |
|
|
on an average more
→of the species of
large genera
than
small genera; and the varying species of the large genera present a greater number of varieties. We have, also, seen that the species, which are the commonest and the most
vary more than
→rare species with restricted ranges. Let (A) be a common, widely-diffused, and varying species, belonging to a genus large in its own country. The
→little fan of
diverging dotted lines of unequal lengths proceeding from (A), may represent its varying offspring. The variations are supposed to be extremely slight, but of the most diversified nature; they are
supposed all to appear simultaneously, but often after long intervals of time; nor are they all supposed to endure for equal periods. Only those variations which are in some way profitable will be preserved or naturally selected. And here the importance of the principle of benefit
derived from divergence of character comes in; for this will generally lead to the most different or divergent variations
by the outer dotted lines) being preserved and accumulated by natural selection. When a dotted line reaches one of the horizontal lines, and is there marked by a small numbered letter, a sufficient amount of variation is supposed to have been accumulated to
→have formed
a fairly well-marked variety, such as would be thought worthy of record in a systematic work. |
|
| The intervals between the horizontal lines in the diagram, may represent each a thousand
→generations; but it would have been better if each had represented ten thousand
After a thousand generations, species (A) is supposed to have produced two fairly well-marked varieties, namely
and
m
1
.
These two varieties will generally
be exposed to the same conditions which made their parents variable,
|
