1888.] Evolution as illustrated by the Geogr. Distrib, of Plants. 285 



Europe and America were more closely allied than is now the case, that 

 the same was true of North and South America, and of Africa and India. 



If this be granted we have no choice as to whether we are to trace 

 existing species to former equally widely diffused species. The 

 problem is thus set as by nature itself. In passing from previous 

 geological times down to the present the phenomenon to be explained 

 is not so much why it is that some modern species are so widely 

 diffused, but why it is that modern species generally have become 

 relatively more localized, why for example the marsupials once 

 abounding elsewhere should now be almost confined to the Australian 

 Area. Whether we adopt the theory of local origin and subsequent 

 migration or that of simultaneous variation the main outlines of the 

 problem are in this respect the same. 



It must be remembered that to advocate the doctrine of simultaneous 

 parallel variation as a cause of the co-existence of the same species in 

 remote areas is not to deny that migration or accidental or intentional 

 transport is a cause also. Where the agency of man comes in, the 

 spread of species by migration is a well-known fact, and it is equally 

 well known that the spores of many plants and the seeds of not a few 

 can be carried great distances by the wind, that some seeds are trans- 

 ported thousands of miles by ocean currents, without losing their 

 vegetative powers, and that birds and even insects must sometimes 

 transport seeds. 



On the other hand too much stress must not be laid on the effect of 

 the infinite variety of surrounding conditions or the differentiation of 

 nascent species. This variety is, it is true, in one sense infinite but in 

 another sense it is very limited. Climate, soil, and rivalry in the 

 struggle for existence may be varied infinitely but only in infinitely 

 small respects while in those general aspects which favour or oppose 

 the success of a species, the variations are by no means so numerous, 

 otherwise the same species would not sometimes flourish over such 

 large areas as they do, and in such vastly different conditions as to 

 soil, climate and competition, and would not be as capable as they are 

 of rapidly overrunning new areas into which they have been acciden- 

 .tally introduced. 



DARWIN'S VIEWS. 



Those who oppose the theory of simultaneous development seem to 



rassume that they have the authority of Darwin on their side. This is 



;by no means clear. What Darwin's opinions were on the points we 

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