338 ON THE ORIGIN OF SPECIES 



Zealand and to Europe, but have not been found in the 

 intermediate tropical seas. 



It should be observed that the northern species and 

 forms found in the southern parts of the southern hemi- 1 

 sphere, and on the mountain-ranges of the intertropical 

 regions, are not arctic, but belong to the northern tern- 4 

 perate zones. As Mr. H. C. Watson has recently re- ) 

 marked, ' In receding from polar towards equatorial ] 

 latitudes, the Alpine or mountain floras really become 

 less and less arctic' Many of the forms living on the 

 mountains of the warmer regions of the earth and in 1 

 the southern hemisphere are of doubtful value, being 

 ranked by some naturalists as specifically distinct, by 

 others as varieties ; but some are certainly identical, • 

 and many, though closely related to northern forms,! 

 must be ranked as distinct species. 



Now let us see what light can be thrown on the fore- 1 

 going facts, on the belief, supported as it is by a large j 

 body of geological evidence, that the whole world, or 

 a large part of it, was during the Glacial period simul- 

 taneously much colder than at present. The Glacial 

 period, as measured by years, must have been very 

 long ; and when we remember over what vast spaces 

 some naturalised plants and animals have spread within 

 a few centuries, this period will have been ample for 

 any amount of migration. As the cold came slowly on, 

 all the tropical plants and other productions will have 

 retreated from both sides towards the equator, followed 

 in the rear by the temperate productions, and these by 

 the arctic ; but with the latter we are not now concerned. 

 The tropical plants probably suffered much extinction ; 

 how much no one can say ; perhaps formerly the 

 tropics supported as many species as we see at the pre- 

 sent day crowded together at the Cape of Good Hope, 

 and in parts of temperate Australia. As we know that 

 many tropical plants and animals can withstand a con- 

 siderable amount of cold, many might have escaped ex- 

 termination during a moderate fall of temperature, 

 more especially by escaping into the lowest, most pro- 

 tected, and warmest districts. But the great fact to 



