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. 
The 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 their descendants 
have spread to the regions they now inhabit. In the numerous 
cases in which the same group now occupies countries 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 their 
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, 
