X 
INTRODUCTION. 
been deemed useful, in preventing the violent action of the coats of the 
stomach against each other, and some have considered them necessary to 
destroy the germinating principle in the grain ; but we would suggest, that 
they are taken into the stomach, to thicken its sides. The firm membrane 
which covers its interior coat is a production of the cuticula, and possesses 
similar properties, as it grows thicker and firmer from attrition. From this 
peculiar quality, the sides of the stomach of these birds, so far from 
suffering from their amazing friction against each other, and the pebbles, 
and their food, increase in strength and thickness,® in some proportion to 
the action of their mechanical pressure. Some of the pebbles are 
* The observations of Spallanzani help to confirm this assertion : for he states, that the stomach 
of a pigeon, in which a rough, unpolished garnet had remained a month, became three times as 
thick 2i.s it commonly is. This ought to have led him to a similar conclusion. The various expe- 
riments which I have made, tend to corroborate this circumstance. All those fowls, which have 
retained large pebbles in their stomachs, even for the short space of ten days, have uniformly had 
them increased in thickness, beyond those of the same brood, the stomachs of which have not 
been acted upon by similar pebbles. In the Appendix to this work will be given, the compa- 
rative results of various attempts to elucidate this question •, but, when Spallanzani asserts that 
birds with muscular stomachs picked up pebbles from accident and not from choice, he certainly 
errs. For, whatever be their real utility, whether they do, or do not, assist digestion, or prevent 
the violent action of the coats of the stomach against each other, destroy the germinating prin- 
ciple of grain, or render by friction the internal coat of the gizzard thicker, and firmer, there 
are facts, in the economy of birds, which irrefragably prove, that they seek for them, and 
swallow them intentionally. I have found them in the stomachs of the Cole, and long-tailed 
Titmice, birds rarely seen on the ground, which most probably descended for that purpose, 
especially the latter, which, uniformly hunts for its food on trees ; and I have discovered them in 
the stomachs of the Nuthatch, and Creeper, birds still more rarely seen on the ground. From 
the gizzard of the Water Hen I have frequently taken stones, above a quarter of an ounce in 
weight, which, from their size, no one can reasonably presume were swallowed by accident. On 
the other hand, I have at times dissected the stomachs of Missel thrushes. Titlarks and Wag- 
tails, birds almost perpetually on the ground, and also those of the Whinchat and Stonechat, 
in which, no pebbles could, by the nicest search, be found. Some of the advocates for the 
utility of pebbles in grinding the food have asserted, that the bird seeks for the most rugged. This 
Is not the fact. I have in the dissection of the stomachs of insectivorous birds, often found 
nothing but smooth grains of gravelly sand. 
