BIRDS AND NATURE 
13 
Consequently the footy just as the beak or wing, creates 
a system of work corresponding to the phenomena of nature, 
which, while essential to the subsistence of each several bird, 
enables its owner to contribute its due share to the work of 
Nature's household. 
That part of the internal organism of birds which belongs 
to this introduction, viz. the digestive organs or, in other 
words, the stomach, is also not uniform. On the one hand 
we have the tubelike stomach which in its simplicity is 
really only an enlargement of the alimentary canal or 
oesophagus, on the other the muscular stomach which by 
stones and gravel swallowed for the purpose actually grinds 
hard food that has been slightly softened in the craw. 
This multiform and varied collection of organic structures 
is in itself enough to point out the variety of foods: and as 
the quantity of each several food is not the same, naturally 
the number of birds living on the respective foods is diverse. 
Consequently a bird may be rare, common or found in large 
quantities. In other words the number of individual birds 
representing each species is proportionate to the quantity of 
that particular food on which the species is dependent for 
its subsistence; and decreases or increases in proportion as 
the supply of food is deficient or copious. 
The latter fact presents us with two sequences: firstly, 
Nature herself, if intact, does not recognise either useful or 
noxious birds, but regulates the number of individual birds 
in accordance with the order and conditions of their life, 
this regulation being, to use a modern term, automatic: on 
the other hand, where the ordinary conditions of Nature 
change, the proportion of bird species changes in accordance 
with variations in the supply of nourishment. 
These facts account for the continual rarety of some species 
and the abundance of others ; for the periodical appearance of 
