MUSEUM OF COMPARATIVE ZOOLOGY. 377 



The uniform character of both the flora and the fauna throughout tlie 

 arctic zone is one of the most striking onto-geographical features thus 

 far known, and one of primary importance, especially when taken in 

 connection with its relation to the fauna? and flora? of more southern 

 latitudes. Not less significant is the fact that in the temperate zone 

 there is still a prevalence of identical forms in each of the three north- 

 ern continents, where the resemblance of the animals and plants of 

 either continent to those of the others is far greater than is the resem- 

 blance of those of the temperate regions of either continent to those of 

 the tropical portions of the same continent. 



Different animals and plants, as every one recognizes, are differently 

 limited in respect to their geographical range. A small proportion of 

 the species are almost or quite cosmopolitan ; others range over the 

 greater part of the northern hemisphere, finding their southern limit of 

 Mstribution near the tropics. A few are exclusively arctic, or range 

 only over the arctic and cold-temperate zones. Many are limited to the 

 temperate zone, throughout nearly the whole of which they find a con- 

 genial home. A large number can only exist within the tropics, often 



limits of distribution and isothermal lines, they following them in all their numerous 

 undulations, sweeping northward in the valleys and southward along the sides of 

 mountain ranges. The occurrence on isolated alpine summits of species existing at a 

 lower level only far to the northward, is of itself suggestive of the powerful influence 

 temperature has on the distribution of animals and plants. In the northern hemisphere 

 a northern fauna and flora everywhere extends along the mountains hundreds of miles 

 to the southward of their respective limits in the adjoining plains and valleys. Various 

 other causes have, of course, a greater or less influence in determining the range of spe- 

 cies, but none other, on the land areas, humidity perhaps alone excepted, is nearly so 

 potent. The want of conformity of isothermal lines with parallels of latitude has doubt- 

 less led to confusion in regard to this subject, since vain attempts have often been made 

 to circumscribe the botanical and zoological zones by the latter. 



Differences of temperature evidently explain many of the otherwise seeminglv inex- 

 plicable sudden transitions in the faunae and florae of adjoining regions, especially in re- 

 gard to the marine animals and plants, temperature forming a strong barrier to the com- 

 mingling of species inhabiting the waters of opposite sides of peninsulas having a north 

 and south trend, or such long narrow points of land as terminate the South American ami 

 African continents. Those of the one side cannot pass to the other without passing 

 through a zone of colder water than their organization will allow them to sustain. The 

 isotherms of the continents are widely deflected by the irregularities of the surface of the 

 land, running nearly straight and parallel across level areas; but in mountainous districts 

 they bend abruptly northward or southward, following along the sides of mountains in- 

 stead of crossing them. In the same manner are species, and fauna? and florae, limited, 

 — a coincidence clearly indicative of th« strong influence climates exert in determining 

 their geographical limits. 



