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continue to exist to 1859 1860 1861 1866
exist at 1869
have survived to 1872

the very process of 1859 1860 1861 1866
their successors, which were gradually rendered more 1869 1872

animal, by some modification of its structure, for 1859 1860 1861 1866
structure of the animal to 1869 1872

But it is 1859 1860 1861 1866
It is, however, 1869 1872

grades of the structure will seldom continue to exist to the present day, for they will have been supplanted by the very process of
perfect
perfection
through natural selection. Furthermore, we may conclude that
transi- tional
transitional
states
grades
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;
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
both of
of
diversified and of changed habits in the individuals of the same species.
In
When
either case
occurs,
occurs,
it would be easy for natural selection to
adapt
fit
the animal, by some modification of its structure, for its changed habits, or exclusively
to
for
one of its several
different
different
habits. But it is difficult to
decide,
tell,
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
occurring
change
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