GEOGEAPHICAL DISTRIBUTION. 53 



that there shall appear to be out-lying points. When, how- 

 ever, 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." — {Forbes.) 



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. One of these relates to the genus Mitra, of which there 



are 420 species ; it has its metropolis in the Philippine Islands, 



I and extends by the Eed 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 Fanopcea, 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 cii'cling wave.* 



,. 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 



divisions 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 life have emanated. 



Boundaries of Natural History Frovinces. 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 migrations 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 variety 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- 



* Tlie most sfriking and conclusive instances may be met with in the distribution ol 

 the highest classes of vertebrate animals. 



