LAWS OF ANIMAL DISPERSION. 
279 
to the production of certain modifications in the 
distribution of animals within a comparatively 
small compass, while the primary laws influencing 
their situation on the earth are uninterrupted, and, 
as it were, overrule the others. In confirmation of 
the supposition that the gradual lessening of num¬ 
bers, and gradual deterioration in size and other 
qualities of individuals of a species as we recede 
from their metropolis, depend on a primary law, we 
see the same rule applied to entire tribes and 
classes of animals in numerous instances. If 
secondary causes, such as food, or climate, deter¬ 
mined the limits of species, it would not be found 
that the verge of the range of one species was the 
principal seat of another possessing similar endow¬ 
ments and organization, and feeding for the most 
part similarly. The reasons or causes then of this 
peculiar law or ordinance of Nature are hidden 
from us. The great seat of the feline tribe is in the 
tropical regions, and we see the species there found 
gradually diminishing in number of individuals as 
we advance northward. We see also that the in¬ 
dividuals situated at the outskirts of this great 
metropolis of rapacious creatures are diminished 
in their size and bodily vigour, and that their 
ferocity has suffered decrease. The place of this 
tropical series is now supplied by a new set, and a 
third still more northwardly may without exagger¬ 
ation or difficulty be detected, each undergoing 
within its own limits the same gradual diminution 
and deterioration. Eventually, if we compare the 
contents of the two opposite points with regard to 
this tribe, the difference becomes remarkable; w r e 
find the species few, the individuals also few, their 
size small, and their vigour and ferocity greatly 
reduced at their northern limit, while at the point 
where we commenced, these features are totally 
reversed. The principal seat of the cetaceous 
