272 



THEOKIES AS TO THE NATUEE OF SPECIES. [Ch. XXXV. 



sometimes appropriated to one genus^ as to the whale 



exam 



111 the exploring expeditions to the arctic and antarctic 



pnlse, star-tish^ and 



Sim 



as crustaceans, moUnsks, ser- 



together with plants of the 



(DiatomacGw) y have been fonnd at depths 



sometimes 



temperature 



freezing-point. Even though we concede, therefore, that the 

 >'eographical 



marine snecies mav be more 



m 



►-eneral than that of the terrestrial (the temperature of 



more 



migrations 



seems 



outnumber the inhabitants of the land. 



Without insisting on this point, it may be safe to assume, 

 that, exclusive of microscopic beings, there are between one 

 and two millions of species now inhabiting the terraqueous 

 globe ; so that if onlj one of these were to become extinct 

 annually, and one new one were to be every year called into 

 being, much more than a million of years might be required 

 to bring about a complete revolution in organic life. 



I have never ventured to hazard any precise hypothesis as to 

 the probable rate of change ; but none will deny that when the 

 annual birth and the annual death of one species on the 

 globe was proposed as a mere speculation, this at least was 

 to imagine no slight degree of instability in the animate 

 creation. If we divide the surface of the earth into twenty 

 regions of equal area, one of these might comprehend a space 

 of land and water about equal in dimensions to Europe, and 

 might contain a twentieth part of the million of species which 

 may be assumed to exist in the animal kingdom. In this 

 region one species only would, according to the rate of mor- 

 tality before assumed, perish in twenty years, or only five out 

 of fifty thousand in the course of a century. But as a consider- 

 able proportion of the Avhole would belong to the aquatic 

 classes, with which we have a very imperfect acquaintance, 



them from 



number 



I 



' r 



i 



f 



Cii.> 



-r""^ 



K^' 





hi 



tl 



gnia 



11' 



perli'^P 

 more tl 



in a res 



m 



It is 



an area. 

 Fi-anee. 



it woiil( 

 of' 



birtlian 



the anil 

 from til 



one 



1- 



to pi: 



forward 



special 



dnctic 



old 



f 



I 



ion 



Ollt. 



■ S 



nessed i 



Thegrei 



succeede 

 tliat the 



it 



^asu( 



^^e spec 



'K 



U 



