122 THE THEORY OF EVOLUTION 



of mischief and, further, how excellent are the reasons 

 for the modern legislation which forbids the impor- 

 tation of live animals into the United States, except 

 by permission of the Secretary of Agriculture. No 

 one can tell in advance what plague he may be 

 bringing in. 



These facts also prove definitely that animals 

 and plants may flourish in many regions beside their 

 native homes and that the absence of any organism 

 is not to be explained by assuming its unfitness. 

 Such considerations might easily lead us to conclude 

 that the whole subject of distribution was a meaning- 

 less hodge-podge of uncorrelated facts, which defied 

 explanation and, yet, if there be any truth in the 

 theory of evolution, it must afford an explanation, 

 or else its claims upon our acceptance will be very 

 greatly weakened. The fundamental postulate of 

 the theory is the unbroken continuity of life from its 

 first appearance on the earth and therefore the 

 present arrangement of living things must be the 

 outcome of an unimaginably long series of past 

 changes, not only those produced by the evolutionary 

 process, but also the countless changes of climate 

 and geography which have continually altered the 

 possibility of migration in this or that direction. 

 The great extinctions, which have from time to time 

 devastated whole regions, have also played a part in 

 bringing about the present order of things. At the 

 end of the Pleistocene epoch, immediately preceding 

 our own, North America lost a host of large mam- 



