anterior perforations vary greatly in development. The degree of
convexity of the posterior part of the sternum differs much, being
sometimes almost perfectly flat. The manubrium is rather more prominent
in some individuals than in others, and the pore immediately under it
varies greatly in size.

_Correlation of Growth._--By this term I mean that the whole organisation
is so connected, that when one part varies, other {168} parts vary; but
which of two correlated variations ought to be looked at as the cause and
which as the effect, or whether both result from some common cause, we can
seldom or never tell. The point of interest for us is that, when fanciers,
by the continued selection of slight variations, have largely modified one
part, they often unintentionally produce other modifications. For instance,
the beak is readily acted on by selection, and, with its increased or
diminished length, the tongue increases or diminishes, but not in due
proportion; for, in a barb and short-faced tumbler, both of which have very
short beaks, the tongue, taking the rock-pigeon as the standard of
comparison, was proportionally not shortened enough, whilst in two carriers
and in a runt the tongue, proportionally with the beak, was not lengthened
enough. Thus, in a first-rate English carrier, in which the beak from the
tip to the feathered base was exactly thrice as long as in a first-rate
short-faced tumbler, the tongue was only a little more than twice as long.
But the tongue varies in length independently of the beak: thus, in a
carrier with a beak 1.2 inch in length, the tongue was .67 in length;
whilst in a runt which equalled the carrier in length of body and in
stretch of wings from tip to tip, the beak was .92 whilst the tongue was
.73 of an inch in length, so that the tongue was actually longer than in
the carrier with its long beak. The tongue of the runt was also very broad
at the root. Of two runts, one had its beak longer by .23 of an inch,
whilst its tongue was shorter by .14 than in the other.

With the increased or diminished length of the beak the length of the slit
forming the external orifice of the nostrils varies, but not in due
proportion, for, taking the rock-pigeon as the standard, the orifice in a
short-faced tumbler was not shortened in due proportion with its very short
beak. On the other hand (and this could not have been anticipated), the
orifice in three English carriers, in the Bagadott