ALLEN ON GEOGRAPHICAL DISTRIBUTION OF MAMMALS. 317 



uents and oceans did not greatly differ from their present form, and the 

 former, back to the time of the Devonian formation, were never so com- 

 Ijletely submerged as to be replaced by oceans comparable in depth with 

 our Atlantic and Pacific." * " This curious fact," he says again, " of the 

 almost perfect continuity of all the great masses of land, notwithstand- 

 ing their extremely irregular shape and distribution, is no doubt depend- 

 ent on the [geological] circumstances just alluded to; that the great 

 depth of the oceans and the slowness of the process of upheaval, has 

 almost always produced the new lands close to, or actually connected 

 with, pre-existing lands ; and this has necessarily led to a much greater 

 uniformity in the distribution of organic forms, than would have pre- 

 vailed had the continents been more completely isolated. from each other. 

 . . . the whole land is almost continuous. It consists essentially of 

 only three masses : the American, the Asia- African, and the Australian. 

 The two former are only separated by thirty-six miles of shallow sea at 

 Behring's Straits, so that it is possible to go from Cape Horn to Singa- 

 pore or the Cape of Good Hqpe without ever being out of sight of land ; 

 and owing to the intervention of the numerous islands of the Malay 

 Archipelago the journey might be continued under the same conditions 

 as far as Melbourne and Hobart Town."t The close proximity of the 

 great land-masses in the Arctic regions is a fact to be kept in mind in 

 any discus'sion of the distribution of life in the northern hemisphere, 

 and also the fact that in Tertiary times the connection was almost indis- 

 putably more intimate than it is now. 



and even mammals and birds are greatly affected, and even some are mainly controlled,' 

 in their range by the presence or absence of forests, the distribution of which is so inti- 

 mately connected with climate. The reptiles, unlike mammals and birds, are quickly 

 influenced by changes of temperature, and are unable to exist in the colder parts of 

 the earth. Amphibians also require a moderately warm, or at least temperate, climate, 

 and though ranging beyond the true reptiles become reduced to a few types in the cold- 

 temperate latitudes, beyond which they wholly disappear. Fluviatile and terrestrial 

 mollusks are also exceedingly susceptible to changes in the conditions of life that affect 

 but slightly either insects or vertebrates, especially the two higher classes of the latter, 

 even the geological character of a country having a powerful influence upon their dis- 

 tribution, as well as affecting their size and the thickness of their calcareous covering. 

 While the mammalia are able to survive changes that would exterminate reptiles and 

 amphibians, and are somewhat independent of the influences that govern the existence 

 of many insects and mollusks, their fossil remains must give, for this reason, a less 

 minute record of past geological and climatic changes than either the lower classes of 

 vertebrates, the mollusca, or the insects, and afford a far less detailed record than plants. 

 Among mammals sometimes the same species, and often the same genus, has a range 

 extending from the Arctic regions to the warm-temperate or subtropical latitudes, thus 

 showing an adaptability to varied conditions of existence not exhibited by the lower 

 vertebrates, or by mollusks or ijlants. While their lack of exceptional means of dis- 

 persal and their superiority to forces of restriction that limit many groups of animals 

 Tender them highly useful as a standard of reference in respect to present life-regions, 

 the latter necessarily detracts from their importance as a medium of geological record, 

 so far at least as regards the minuter details. 



* Report of a Lecture before the Royal Geographical Society, in Geogr. Mag., vol. iv, 

 .August, 1877, p. 22L 



t Geogr. Dist. Anim., vol. i, p. 37. 



