GEOGKAPHICAL DISTRIBUTION. 53 



that there shall appear to be out-lying points. "WTien, 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, 

 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 Bufifon, 

 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 thi^ive 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 sfrikinp and conclusive instances may be met with in the distribution of 

 the highest classes of vertebrate animals. 



