SUMMARY AND CONCLUSION 309 



where the land masses are greatest north of the Equator, 

 we should similarly expect to find, and do find, the 

 most important and extensive assemblages of species 

 and types presenting the greatest amount of differences 

 from such assemblages of types and species dwelling on 

 the land masses south of the Equator ; and these assem- 

 blages will to a great extent be homogeneous or other- 

 wise in proportion to longitudinal continuity equatorially 

 of such Northern Hemisphere areas. To a great extent 

 this is quite irrespective of existing equatorial land areas 

 connecting the Northern and Southern Hemispheres 

 respectively. 



In my opinion we have hitherto attacked the problem 

 of migration and geographical dispersal at the wrong 

 end. We have regarded Glacial Epochs, climatic 

 changes, and physical mutations, as grand distributors 

 of species, compelling Southern Emigration in one 

 hemisphere. Northern Emigration in the other, rather 

 than as vast exterminating influences which have re- 

 peatedly cleared one dominant flora and fauna after 

 another from all the areas affected, the only surviving 

 forms being those whose bases or areas in which 

 Reproduction took place were beyond the fatal limits of 

 glacial or other influence. The enormous scope for evo- 

 lution in the oft-repeated march of life from the tropics 

 or temperate zones towards the Poles combined with 

 physical change must be apparent to every reader. 

 Such periodical emigrations to a great extent account 

 not only for the origin, but for the geological sequence 

 of species. This Law of Dispersal will explain why in 

 past ages we meet with a great extension of range of 

 forms which are now limited to small areas — eloquent 



