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These frigid conditions were preceded b}- a warmer Pliocene 

 period during which deciduous trees and evergreens flourished 

 within 10 degrees of the present position of the Pole, and it 

 is probable that similar alternations of climatal severity have 

 occurred from time to time in the history of the world, yet 

 all these changes would be so excessively slow that if any powers 

 of adaptability be conceded to organised life, we are compelled 

 to allow that most of the less severe changes would have been 

 guarded against by suitable modifications of the affected organ- 

 isms in response to the gradually changing conditions to which 

 they were subjected. 



But without wishing to minimise a reasonable interpretation 

 of the effects of these extreme conditions, all of which are of 

 a more or less temporary character and do not vitally affect 

 the principles involved in the distribution of life, we must, I 

 think, come to the conclusion that a greater and more far-reaching 

 cause has not only brought about the present distribution of 

 life, but has been equally potent in all ages, and that cause is 

 undoubtedly the increasing and unending struggle for existence 

 between the various species and groups, a struggle that in- 

 variably results in the extermination or expulsion of the weaker 

 and more primitive competitors and their eventual restriction 

 to isolated, remote, or inhospitable regions : thus Arctic, montane, 

 desert, or other trying conditions of life are not adopted from 

 choice, but the organisms now living under such conditions 

 have been compelled by the stress of competition to retreat 

 thereto and adapt themselves to the cold or barren stations 

 not occupied by the stronger races. The routes by which the 

 exodus of improved forms from the active evolutionary area 

 in Europe takes place towards Asia, America, and Australasia 

 is by way of South Russia mainly through the stretch of country 

 lying between the southern boundary of the Boreal proxince 

 of Milachevitch and the northern boundary of the Pontic 

 province of Drouet, and that this is the probably true path of 

 emigrant mollusca is demonstrated by the reliable records es- 

 tablishing the direction and progress of the eastern dispersal 

 of the most dominant Helices, H. pomaiia, H. nemoralis, and 

 H. hortensis (PI. VII, fig. 3). 



The correctness of this view is now accepted by one of the 



