GEOGRAPHICAL DISTRIBUTION. 58 
that there shall appear to be out-lying points. When, how- 
eyer, the history of a natural genus shall have been traced 
equally through its extension in time and space, it is not 
impossible that the area, considered in the abstract, will be 
found to be necessarily unique.” —(/orbes.) 
To illustrate the doctrine of the unity of generic areas Professor 
Forbes has given several examples, showing that some of the 
most exceptional cases admit of explanation and confirm the 
rule. Oné of these relates to the genus Mitra, of which there 
are 420 species; it has its metropolis in the Philippine Islands, 
and extends by the Red Sea to the Mediterranean and West 
Africa, the species becoming few, small, and obscure. Far 
away from the rest a single species is found on the coast of 
Greenland. But this very shell occurs fossil in Ireland along 
with another mitra now living in the Mediterranean. Another 
case is presented by the genus Panopca, of which the eleven 
living species are widely separated. Of this genus above 100 
fossil species are known, distributed over many places within 
the wide area, on whose margin the relics of this ancient form 
of life seem to linger like the last ripple of a circling waye.* 
According to this view the specific centres are scattered 
thickly over the whole surface of the globe ; those of the genera 
more thinly distributed; and the points of origin of the large 
groups become fewer in succession, until we have to estimate 
the probable position or scene of creation of the primary 
diyisions themselves; and are led to speculate whether there 
may not have been some common focus—the centre of centres 
—from which the first and greatest types of hfe have emanated. 
Boundaries of Natural History Provinces. The land provinces 
are separated by lofty mountains, deserts, seas, and climates; 
whilst the seas are divided by continents and influenced by the 
physical character of coast-lines, by climates and currents. 
These ‘‘natural barriers,” as they were called by Buffon, 
retard or altogether prevent the enn of species in 
particular directions. 
Influence of Climate. Diversity of climate has been the 
popular explanation of most of the phenomena of geographical 
distribution, because it is so well known that some species 
require a'tropical amount of warmth, whilst others can endure 
a great yariety of temperature, and some only thrive amidst 
the rigours of the arctic regions. The character of the vegeta- 
tion of the zones of latitude has been sketched by Baron Hum- 
* The most sfriking and conclusive instances may be met with in the distribution of 
the highest classes of vertebrate animals. 
