Chapter VI 



PARALLELISM IN THE GEOGRAPHICAL DISTRIBUTION 



OF PLANTS AND ANIMALS AND CORRELATION 



BETWEEN THE DISTRIBUTION OF PARASITES 



AND THAT OF THEIR PLANT HOSTS 



The difficulties involved in retracing the history of the geographical 

 distribution of plants, considering the paucity of paleobotanic data, 

 make it necessary, in addition to the direct study of the areas of 

 species, to make use of all possible indirect methods that may aid in 

 solving this problem. Among such methods the chief is a comparison 

 with data on the geographical distribution of animals. 



The distribution of plants and animals, both at the present time and 

 in former geological epochs, is in many ways interdependent. Eco- 

 logical conditions, past changes in climate and in the connections and 

 configurations of continents, are reflected, to a greater or less extent, 

 in the distribution of all living organisms. Consequently, the geogra- 

 phy of plants and animals really should constitute a single science, 

 biogeography. Such a science is now only a dream for the future, but 

 it is to be hoped that it will some day be realized. If now the study of 

 the geography of the floras and faunas of the globe is separated, this is 

 the regrettable result of the scope of modern knowledge, which has out- 

 grown the possibility of its mastery by a single investigator. This need 

 for specialization hampers to some extent the study of nature as a uni- 

 fied whole. 



It is, nevertheless, of utmost importance that phytogeographers and 

 zoogeographers should co-operate closely in their work. The agreement 

 of data in both branches of biogeography, the similarities in the distri- 

 bution of plants and animals, the occurrence of the same discontinu- 

 ities in their areas, and the identity of the centers of origin of floras 

 and faunas leave no doubt as to the existence of historical causes for 

 these phenomena and give a sound basis for phyto- and zoo-geographi- 

 cal conclusions. In the present chapter we do not aim to go into this 

 phase of our general problem in detail. We wish merely to give a few 

 illustrations of parallelisms in the distribution of plants and animals, 

 which will make clear the importance of checking conclusions made on 

 the basis of a direct study of plant distribution by comparison with 

 zoogeographical data. 



As a second indirect method, we wish to stress the importance of a 

 study of parasites — both of plant and animal origin, connected during 

 the course of their life cycles with definite plants — which may provide 

 very valuable aid in establishing the history of the distribution of their 

 plant hosts. The parallel study of both reveals their interrelations, 

 paths of migration, and also their past areas of distribution, often 

 indicating that there formerly existed a different distribution of the dry 

 lands and climatic zones of the globe. 



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