Chapter VI — 95 — Parallelisms in Distribution 



By the aid of such indirect methods it frequently becomes possible 

 to establish the past distribution of plants in cases where ordinary 

 methods — data on present distribution and paleobotany — fail to pro- 

 vide sufficient clues. 



Parallelism in the Distribution of Plants and Animals: — Similar 

 features in the distribution of plants and animals arise from three cir- 

 cumstances: (j) the dependence of animals on plants as a source of food 

 and of plants on animals as agents in the transfer of pollen and the 

 dispersal of fruits and seeds; (2) the effect of identical ecological factors 

 and the habitation of common biocenters (this constitutes the basis for 

 the view held by some investigators that there do not exist separate 

 plant and animal associations but only common, complex bio-associ- 

 ations); and (j) historical causes, such as changes in climate and in 

 the position of the continents, having a like effect on the areas of 

 plants and animals. 



In connection with the theme of our present manual of particular 

 interest are those similarities in distribution induced by historical 

 causes. These are expressed in the frequent occurrence of rehc retreats 

 common for both plants and animals. Moreover, sometimes a plant 

 association, e.g., a forest, may be a relic retreat for animals, although it 

 itself is not rehc in nature. This is confirmed by the fact that a 

 number of plants and animals have the same centers of development of 

 their areas and also by the similarity of their discontinuous areas. 

 This may be seen from the following examples. According to Geptner 

 (1936), twenty-one species of fresh-water fish of the British Isles, for 

 which the sea constitutes an insurmountable barrier, are identical with 

 fish hving in the Rhine and rivers of northwestern France. The occur- 

 rence of the same plants in Great Britain and along the banks of the 

 Rhine is a parallel phenomenon, and is explained by the fact that the 

 Rhine, at the time of the post-glacial connection between the British 

 Isles and the mainland, flowed through England and had its mouth in 

 the northern part of Norfolk County. 



On a plateau in the central part of the Sahara there is found fauna 

 of the Mediterranean type, similar to the fauna of the Atlas Mts., 

 despite the fact that deserts now separate them. Similar discontinuous 

 areas are characteristic of the floras of the mountain chains of Africa. 

 The absence of marked changes in climate since the beginning of the 

 Tertiary period in the region of the Indian Ocean has resulted in the 

 presence on its islands of an exceedingly rich flora, preserving Tertiary 

 features, and also of a very rich fauna, including a shore fauna that 

 arose from the fauna of Tethys. There are animals that have bipolar 

 areas with a break in the tropical zone, just as in the case of plants. 



CocKERELL (1932) has pointed out an interesting paralleHsm be- 

 tween the discontinuous area of a genus of bees, Hesperapis — distrib- 

 uted in arid districts of southwestern U.S.A. and also, under similar 

 habitat conditions, in South Africa — and an analogous area of a genus 

 of plants, Menodora, of the family Olcaceae. This genus is distributed 

 in semi-arid districts of the southwestern part of North America (in- 

 cluding Mexico and the southwestern States of the U.S.A.), in central 

 and southern South America, and then, after another break, in the 



