| → away 1861 1866 1869 |
| with its beak 1872 |
|
| → each slight variation of beak, 1861 1866 |
| all the slight individual variations in the shape of the beak, which were 1869 1872 |
|
| → from the want of other food, or the preservation of chance 1861 1866 |
| or spontaneous 1869 1872 |
|
| → more 1861 1866 |
| to become more 1869 1872 |
|
| → but 1861 1866 |
| or taste; but 1869 1872 |
|
|
entirely
on the assumption that the changes in
and structure are abrupt. To take as an illustration the case of the larger titmouse (Parus major) alluded to in
chapter: this bird often holds the seeds of the yew between its feet on a branch, and hammers
→away
till it gets
the kernel. Now what special difficulty would there be in natural selection preserving
→each slight variation of beak,
better and better adapted to break open
until a beak was formed, as well constructed for this purpose as that of the nuthatch, at the same time that
habit, or
→from the want of other food, or the preservation of chance
variations of taste,
the bird
→more
and more of a seed-eater? In this case the beak is supposed to be slowly modified by natural selection, subsequently to, but in accordance with, slowly changing
→but
let the feet of the titmouse vary and grow larger from correlation with the beak, or from any other unknown cause, and
it
improbable that such larger feet
lead the bird to climb more
until it acquired
the remarkable climbing instinct and
of the
In this case a gradual change of structure is supposed to lead to changed instinctive
To take one more case: few instincts are more remarkable than that which leads the swift of the Eastern Islands to make its nest wholly of inspissated saliva. Some birds build their nests of mud, believed to be moistened with saliva; and one of the swifts of North America makes its nest (as I have seen) of sticks agglutinated with saliva, and even with flakes of this substance. Is it then very improbable that the natural selection of individual swifts, which secreted more and more saliva, should at last produce a species with instincts leading it to neglect other materials, and to make its nest exclusively of inspissated saliva? And
|
