and the powers of
flight,--so-called "homing" birds having enormous flying powers;*
([Footnote] *The "Carrier," I learn from Mr. Tegetmeier, does not
'carry'; a high-bred bird of this breed being but a poor flier. The
birds which fly long distances, and come home,--"homing" birds,--and are
consequently used as carriers, are not "carriers" in the fancy sense.)
while, on the other hand, the little Tumbler is so called because of its
extraordinary faculty of turning head over heels in the air, instead of
pursuing a direct course. And, lastly, the dispositions and voices of
the birds may vary. Thus the case of the pigeons shows you that there
is hardly a single particular,--whether of instinct, or habit, or
bony structure, or of plumage,--of either the internal economy or the
external shape, in which some variation or change may not take place,
which, by selective breeding, may become perpetuated, and form the
foundation of, and give rise to, a new race.

If you carry in your mind's eye these four varieties of pigeons, you
will bear with you as good a notion as you can have, perhaps, of the
enormous extent to which a deviation from a primitive type may be
carried by means of this process of selective breeding.

End of The Perpetuation of Living Beings.

THE CONDITIONS OF EXISTENCE AS AFFECTING THE PERPETUATION OF LIVING
BEINGS.

In the last Lecture I endeavoured to prove to you that, while, as a
general rule, organic beings tend to reproduce their kind, there is
in them, also, a constantly recurring tendency to vary--to vary to a
greater or to a less extent. Such a variety, I pointed out to you, might
arise from causes which we do not understand; we therefore called it
spontaneous; and it might come into existence as a definite and marked
thing, without any gradations between itself and the form which preceded
it. I further pointed out, that such a variety having once arisen,
might be perpetuated to some extent, and indeed to a very marked extent,
without any direct interference, or without any exercise of that process
which we called selection. And then I stated further, that by such
selection, when exercised artificially--if you took care to breed only
from those forms which presented the same peculiarities of any variety
which had arisen in this manner--the variation might be perpetuated, as
far as we can see, indefinitely.

The next question, and it is an important one for us, is this: Is there
any limi