278 Prof. Guthrie, LL.B. — On the Subjective Causes q/' [April 25^ 



The former alternative however seems to be fatal to the migration 

 theory. If in any locality a number of individuals of a species A 

 simultaneously and independently vary either per saltum or by degrees, 

 and in the course of many generations into the B form, there seems 

 to be no valid reason why this process should be confined to one 

 locality of the seat or seats of the A species. 



If independent simultaneous and similar variation be admitted at 

 all, it seems impossible to deny that it may take place in more than 

 one locality either in the same or different floral areas. The denial 

 of this possibility seems necessarily to involve the hypothesis of 

 species being derived from a single ancestor or ancestral pair. 



On the migration theory the existence of the same species in remote 

 areas is accounted for either on the hypothesis of the actual con- 

 veyance of seeds or plants over the intervening regions by birds, ocean' 

 currents, or otherwise, or on the hypothesis favoured by Darwin and^ 

 Hooker that the two regions where the common species exist were 

 once united with each other or at different times with some third 

 region by similar climatic and other conditions and thus formed a 

 single floral area, which has since become severed by change in the 

 climate and physical conditions of intermediate regions. 



The existence of distinct floral areas under similar physical con- 

 ditions is not absolutely inconsistent with this theory of the origin 

 of species but presents many difficulties. 



European species can and do thrive in America ; and vice versa, if 

 their migration on a large scale has been possible between Europe 

 and the Antarctic regions, it is difficult to understand why there- 

 should not be a greater community of specific forms in regions so 

 close as Europe, and Northern Asia (which belongs to the European 

 area) and America. 



So also if migration were so efficient a factor in the distribution 

 of species as this theory supposes it is hard to understand why so 

 marked a difference should still continue to exist in the fauna and 

 flora of islands belonging to the Moluccan and Australian areas,, 

 which are only separated from each other by a narrow strait (see 

 Wallace). 



A continuity of physical condition, such as would have connected 

 the Australian Alps and Siberia into one area, would surely have 

 also connected the tropical and sub-tropical regions of Australia- 

 and the Moluccas and thus have left far more distinct traces of its 

 existence. 



