290 
INFLUENCES OF FOOD. 
some species have been gained, and certainly the 
number of individuals has in many cases been in¬ 
creased, and a great variety of alterations effected 
in geographic position. The number of our insects 
and molluscs has certainly been increased, and 
their geographic limits and positions considerably 
interfered with, contracted, or enlarged. Many 
instances in point will be afforded in the sequel. 
Very little need be said on the subject of food 
as affecting the geography of animals. By the 
polity of nature the vegetable world and the series 
of animals are intimately blended and connected : 
an extensive Flora will for the most part imply a 
large proportion of animals, and so likewise the 
weaker creatures draw to them the carnivorous 
tribes. In our county we find an extensive Flora, 
and our woods are numerous and deep ; the series 
of animals also is found very perfect, and the parts 
of it would, if not subjected to our interferences 
and persecutions, be relatively proportionate. Food 
influences the migration of animals to and from 
this country, subject however to the higher in¬ 
fluence of weather. In order to comprehend the 
relative proportions of influence exerted by these 
two causes, we must suppose adverse and propit¬ 
ious cases in point. If food is plentiful and the 
weather intemperate a summer bird of passage will 
forthwith undertake its journey if it be near the 
usual period for migrating; if food is scarce and the 
weather fine, it will also depart; if both circum¬ 
stances are adverse, it will hasten its departure still 
more decidedly ; and if provision be in plenty and 
the weather fine, its stay will be prolonged. Similar 
remarks mightbe made relatively to winter migrants. 
Food has considerable influence in determining the 
other kinds of migration besides the vernal and 
autumnal. It also causes a variety of unusual 
movements in animals, as will appear in detail. 
