STRUCTURE AND GEOGRAPHICAL DISTRIBUTION. 53 



SECTION X. 



IDENTITY OF STRUCTURE OF WIDELY DISTRIBUTED TYPES. 



It is not only when considering the diversification of 

 the animal kingdom within limited geographical areas, 

 that we are called upon in our investigations to admire 

 the unity of plan which its most diversified types exhibit ; 

 the identity of structure of these types is far more sur- 

 prising, when we trace it over a wide range of country 

 and within entirely disconnected areas. Why the animals 

 and plants of North America should present such a strong 

 resemblance to those of Europe and Northern Asia, while 

 those of Australia are so entirely different from those of 

 Africa and South America under the same latitudes, is 

 certainly a problem of great interest, in connection with 

 the study of the influence of physical agents upon the 

 character of animals and plants in different parts of the 

 world. North America certainly does not resemble Eu- 

 rope and Northern Asia, more than parts of Australia 

 resemble certain parts of Africa or of South America ; 

 and, even if a greater difference should be conceded be- 

 tween the latter than between the former, these dispa- 

 rities are in no way commensurate with the difference or 

 similarity of their organized beings, nor in any way 

 rationally dependent one upon the other. Why should 

 the identity of species prevailing in the Arctic not ex- 

 tend to the Temperate zone, when it is as difficult to dis- 

 tinguish many species of this zone, though different, as it is 

 to prove the identity of certain arctic species where the 

 continents converge towards the north ; and when, besides, 

 the species of the two zones mingle to a great extent 

 at their boundaries "? Why are the antarctic species 



