grades of the structure will seldom
continue to exist to continue to exist to 1859 1860 1861 1866 |
exist at 1869 |
have survived to 1872 |
the present day, for they will have been supplanted by
the very process of the very process of 1859 1860 1861 1866 |
their successors, which were gradually rendered more 1869 1872 |
perfection perfection 1859 1860 1861 1866 | perfect 1869 1872 |
through natural selection. Furthermore, we may conclude that
transitional transitional 1860 1861 1866 1869 1872 | transi- tional 1859 |
grades grades 1859 1860 1861 1866 | states 1869 1872 |
between structures fitted for very different habits of life will rarely have been developed at an early period in great numbers and under many subordinate forms. Thus, to return to our imaginary illustration of the
flying-fish, flying-fish, 1859 1860 1861 1869 1872 | flying-fish; 1866 |
it does not seem probable that fishes capable of true flight would have been developed under many subordinate forms, for taking prey of many kinds in many ways, on the land and in the water, until their organs of flight had come to a high stage of perfection, so as to have given them a decided advantage over other animals in the battle for life. Hence the chance of discovering species with transitional grades of structure in a fossil condition will always be less, from their having existed in lesser numbers, than in the case of species with fully developed structures. |
|
I will now give two or three instances
of of 1859 1860 1861 1866 1869 | both of 1872 |
diversified and of changed habits in the individuals of the same species.
When When 1859 1860 1861 1866 | In 1869 1872 |
either case
occurs, occurs, 1859 1860 1861 1866 | occurs, 1869 1872 |
it would be easy for natural selection to
fit fit 1859 1860 1861 1866 | adapt 1869 1872 |
the
animal, by some modification of its structure, for animal, by some modification of its structure, for 1859 1860 1861 1866 |
structure of the animal to 1869 1872 |
its changed habits, or exclusively
for for 1859 1860 1861 1866 | to 1869 1872 |
one of its several
different different 1859 1860 1861 1866 | different 1869 1872 |
habits.
But it is But it is 1859 1860 1861 1866 |
It is, however, 1869 1872 |
difficult to
tell, tell, 1859 1860 1861 1866 | decide, 1869 1872 |
and immaterial for us, whether habits generally change first and structure afterwards; or whether slight modifications of structure lead to changed habits; both probably often
change change 1859 1860 1861 1866 | occurring 1869 1872 |
almost simultaneously. Of cases of changed habits it will suffice merely to allude to that of the many British insects which now feed on exotic plants, or exclusively on artificial substances. Of diversified habits innumerable instances could be given: I have often watched a tyrant flycatcher (Saurophagus sulphuratus) in South America, hovering over one spot
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