124 EVOLUTION AND DOGMA. 



impressed with the divers new and strange organ- 

 isms which are continually presented to his view. 

 The fauna on the opposite sides of mountain chains 

 are often quite unlike, although the conditions of 

 existence may be essentially the same. The animals 

 on the contiguous islands of an archipelago are specif- 

 ically distinct from one another, and generically dif- 

 ferent from the animals on the nearest mainland. 

 The marine fauna on the opposite sides of the 

 Isthmus of Panama, although the conditions of ex- 

 istence on the eastern and western shores are appre- 

 ciably the same, are almost wholly distinct, when, if 

 we considered only their environment, we should 

 expect them to be exactly alike. 



Whithersoever we go, we observe that " barriers 

 of any kind, or obstacles to free migration, are related 

 in a close and important manner to the differences 

 between the productions of various regions. We 

 see this in the great difference in nearly all the ter- 

 restrial productions of the New and Old Worlds, 

 excepting in the northern parts where the land 

 almost joins, and where, under a slightly different 

 climate, there might have been free migration for 

 the northern temperate forms, as there is now for 

 the strictly Arctic productions. We see the same 

 fact in the great difference between the inhabitants of 

 Australia, Africa and South America under the same 

 latitude; for these countries are almost as much 

 isolated from each other as is possible. On each 

 continent, also, we find the same fact ; for on the 

 opposite side of lofty and continuous mountain 

 ranges, of great deserts and even of large rivers, we 



