[17] Carpenter, `Principles of Comparative Physiology,' 1854, p. 690, and
Muller's `Elements of Physiology,' Eng. translat. vol. ii. p. 936.

[18] Mowbray on `Poultry,' 6th edit. 1830, p. 54.

I will give only one other instance of an habitual and
purposeless movement. The Sheldrake (_Tadorna_) feeds on the sands
left uncovered by the tide, and when a worm-cast is discovered,
"it begins patting the ground with its feet, dancing as it were,
over the hole;" and this makes the worm come to the surface.
Now Mr. St. John says, that when his tame Sheldrakes "came to ask
for food, they patted the ground in an impatient and rapid
manner."[19] This therefore may almost be considered as their
expression of hunger. Mr. Bartlett informs me that the Flamingo
and the Kagu (_Rhinochetus jubatus_) when anxious to be fed,
beat the ground with their feet in the same odd manner.
So again Kingfishers, when they catch a fish, always beat
it until it is killed; and in the Zoological Gardens they
always beat the raw meat, with which they are sometimes fed,
before devouring it.


We have now, I think, sufficiently shown the truth of our first Principle,
namely, that when any sensation, desire, dislike, &c., has led during
a long series of generations to some voluntary movement, then a tendency
to the performance of a similar movement will almost certainly be excited,
whenever the same, or any analogous or associated sensation &c., although
very weak, is experienced; notwithstanding that the movement in this