TERTIARY MAMMAL HORIZONS. 47 



into six great regions and into eastern and western divisions or 

 Palaeoga^a and Neogaea, to embrace the Old and New Worlds 

 respectively, a division which has proved to be illogical. 

 This led Darwin's distinguished colleague, Alfred Wallace, 

 to his great work upon the ' Geographical Distribution of 

 Animals ' and the division of the world into life regions ; in 

 which Sclater's scheme was adopted and developed.^ In 1868 

 Huxley divided the world into a northern division, Arctogaea, 

 and a southern division Notogaea to include the Northern and 

 Southern Hemispheres respectively ; this division was a little 

 nearer the truth than Sclater's. Between 1868 and 1890, 

 Sclater, Allen, Newton and Blanford, working upon living 

 birds and mammals, continued this investigation, but it remained 

 for Blanford, in 1890, to prove that the world zoologically 

 should be divided into three great divisions ; an Australian, a 

 South American and a third region, Arctogaea, comprising North 

 America, Europe, Asia and Africa." 



Now it is clear that exactly as our understanding of the re- 

 lations of living animals and plants to each other depends upon 

 their fossil ancestors or upon their palaeontology, so the final test of 

 a scheme of zoological distribution must be a palaeontological test. 

 The animals of various families and orders have either originated in 

 or migrated into their present habitat in past time, so that the 

 geological record as to their order of appearance becomes of 

 first importance. Here again the necessity of an absolutely re- 

 liable correlation time scale such as we are now establishing be- 

 comes evident, for the very first step toward an exact solution oi 

 the problem of past migration is to establish, as far as possible, 

 the faunal parallels upon different continents ; we can then de- 

 termine where certain types of animals first appeared, and dis- 

 tinguish between the autocthonous endemic or native types and 

 the migrant or new types. 



' The history of opinion upon this subject is" fully set forth by Lydekker's val- 

 uable work the " Geographical Distribution of Mammals," published in 1896. 



2 Dr. Theodore Gill has kindly called attention [Science, June 8, 1900) to my 

 oversight of an important paper of his ("On the Geographical Distribution of 

 Fishes," Attn. Mag. iVaf. Hist., 1875, PP- 251-255). He unites South America, 

 Australia and Africa into a single division Eog.-ea, in contrast with C/ENOG.e:a, 

 which includes North America, Eurasia and India. 



