xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 341 



must take account of this condition of things whenever we 

 have to speculate on the possible migrations of organisms 

 between the old and new continents. 



Hie Conditions which have determined Distribution. 



When we endeavour to explain in detail the facts of the 

 existing distribution of organic beings, we are confronted by 

 several preliminary questions, upon the solution of which will 

 depend our treatment of the phenomena presented to us. 

 Upon the theory of descent which we have adopted, all the 

 different species of a genus, as well as all the genera which 

 compose a family or higher group, have descended from some 

 common ancestor, and must therefore, at some remote epoch, 

 have occupied the same area, from which then descendants 

 have spread to the regions they now inhabit. In the numerous 

 cases in which the same group now occupies coiintries separated 

 by oceans or seas, by lofty mountain-chains, by wide deserts, 

 or by inhospitable climates, we have to consider how the 

 migration which must certainly have taken place has been 

 effected. It is possible that during some portion of the time 

 which has elapsed since the origin of the group the inter- 

 posing barriers have not been in existence ; or, on the other 

 hand, the particular organisms we are dealing with may have 

 the power of overpassing the barriers, and thus reaching then 

 present remote dwelling-places. As this is really the funda- 

 mental question of distribution on which the solution of all 

 its more difficult problems depends, we have to inquire, in the 

 first place, what is the nature of, and what are the limits to, the 

 changes of the earth's surface, especially during the Tertiary 

 and latter part of the Secondary periods, as it was during those 

 periods that most of the existing types of the higher animals 

 and plants came into existence ; and, in the next place, what 

 are the extreme limits of the powers of dispersal possessed by 

 the chief groups of animals and plants. We will first consider 

 the question of barriers, more especially those formed by seas 



and oceans. 



The Permanence of Oceans. 



It was formerly a very general belief, even amongst 

 geologists, that the great features of the earth's surface, no less 

 than the smaller ones, were subject to continual mutations, 



