| → should thus in the course of time be produced, it is necessary to believe that when 1861 1866 |
| in any part should be effected, 1869 |
| should be effected in a species, 1872 |
|
| → arisen, it again varies, after perhaps 1861 1866 |
| formed must again, perhaps after 1869 1872 |
|
| → and that its varieties, if favourable, are 1861 1866 |
| vary or present individual differences of the same favourable nature, and these must be 1869 |
| vary or present individual differences of the same favourable nature as before; and these must be 1872 |
|
| → onwards. 1861 1866 |
| onwards step by step. 1869 1872 |
|
| ↑ 2 blocks not present in 1859 1860 1861 1866; present in 1869 1872 |
| Seeing that individual differences of all
kinds
perpetually recur, this can hardly be considered as an unwarrantable assumption.
But whether all this has actually taken place must be judged by
how far the hypothesis accords with and explains the general phenomena of nature.
|
|
| → general phenomena of nature. 1866 |
| ordinary belief that the amount of possible variation is a strictly limited quantity is likewise a simple assumption. 1872 |
|
| → and the black-grouse that of peaty earth, 1859 1860 1861 1866 |
| OMIT 1869 1872 |
|
| → I can see no reason to doubt that 1859 1860 1861 1866 |
| OMIT 1869 1872 |
|
|
In order that any great amount of modification
→should thus in the course of time be produced, it is necessary to believe that when
a variety
once
→arisen, it again varies, after perhaps
a long interval of
→and that its varieties, if favourable, are
again preserved, and so
→onwards.
↑ That varieties more or less different from the parent-stock occasionally arise, few will deny; but that the process of variation should be thus indefinitely prolonged is an assumption, the truth of which must be judged of by how far the hypothesis accords with and explains the general phenomena of nature. On the other hand, the
→general phenomena of nature. On the other hand, the ordinary belief that the amount of possible variation is a strictly limited quantity is
a simple assumption. |
|
| Although natural selection can act only through and for the good of each being, yet characters and structures, which we are apt to consider as of very trifling importance, may thus be acted on. When we see leaf-eating insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour of heather,
→and the black-grouse that of peaty earth,
we must believe that these tints are of service to these birds and insects in preserving them from danger. Grouse, if not destroyed at some period of their lives, would increase in countless numbers; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their
so much so, that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction. Hence
→I can see no reason to doubt that
natural selection might be
effective in giving the proper colour to each kind of grouse, and in keeping that colour, when once acquired, true and constant. Nor ought we to think that the occasional destruction of an animal of any particular colour would produce little effect: we
|
