(IF ZOOLOGICAL FOCI. 145 
There are two theories, which have mainly divided 
the opinions of naturalists. The first supposes that the 
existing races of animals were of simultaneous creation 
at a common centre or focus, from which they have 
spread over the whole earth ; the other considers that 
there were several distinct centres or foci, from which 
the species radiated into — and thus formed, the zoolo- 
gical circles or regions which are now apparent. The 
first supposition, though possible, is seen at once to be 
attended with many physical difficulties, and has but few 
Supporters ; wliile the second, commends itself to the 
minds of almost all. In surveying the zoology of the 
earth, we find groups of distinct animals, of the different 
classes, occupying regions which are defined by natural 
boundaries, and limited to these regions alone ; and the 
discovery of new continents and islands has, in every 
case, revealed animals peculiar to each of them. The 
existence of distinct zoological regions has hence come 
to be an axiom in the philosophy of zoology, and the 
theory of distinct foci of animal origin has thus been 
greatly fortified. But in all the speculations of natural- 
ists on this subject, and in all their attempts to deduce 
the limits of the various regions, from the actual distri- 
bution of animals, attention has been given almost solely 
to the large animals, and mostly to the vertebrata, and 
consequently the regions as laid down by authors, are co- 
extensive with the wide range of these species. It is 
manifest, however, that the ranges of the different classes 
of animals differ very much among themselves, and that 
