178 The Science of Life. 



About fifty years later, Grisebach (1872) admitted the 

 evolution -idea, though without taking full advantage 

 of it. Every species diffuses from its centre of origin, 

 but is met by climatic and topographical limits, which 

 have resulted in the local peculiarities now observable. 

 Moreover, in the course of diffusion new species may 

 arise in consequence of climatic change and spatial 

 isolation. 



The next great step was taken in 1882, when Engler, 

 following Unger's lead, sought to connect the present 

 vegetation with that of Tertiary times, and to show how 

 the known differences might be definitely accounted for 

 by known changes such as the Ice Age and changes of 

 elevation. Drude (1884) also based his system on the 

 past history of the plant world, but for the details he 

 returned to Schouw's statistical method. 



This brief retrospect shows that climate and soil, 

 geological changes, topographical and other boundaries, 

 means of dispersal, original headquarters and past his- 

 tory were gradually recognized as factors determining 

 the present state of affairs. The difficulty is to combine 

 them. 



There is much scientific utility in an ordered map of 

 the distribution of plants and animals over the earth 

 Factors in and through the seas, but it would be a more 

 Distribution, valuable result if we could show how the 

 present distribution has come to be. It is certainly 

 instructive to note the resemblance in the fauna of areas 

 so widely separated as Britain and Japan, the difference 

 in the fauna of areas so near to one another as Florida 

 and the Bahamas, or as Bali and Lombok (the two 

 islands separated by "Wallace's line"), the distinctive- 

 ness of the Australasian fauna, the peculiarly discon- 

 tinuous distribution of tapirs, Camelidae, and Lemurs, 

 and similarly in regard to plants, for these are among 

 the outstanding facts of geographical distribution, but 

 our standard of biological interest was greatly raised 

 by Darwin. The real interest of the facts is only 

 appreciated when we reach some solution of the fac- 

 tors. 



Our knowledge of the factors is still incomplete, and 



