MUSEUM OF COMPARATIVE ZOOLOGY. 251 



evidence already given in the special discussion of each case, but by 

 the fact of the near approximation of their habitats, and by general 

 principles. 



The thirteen species of land mammalia common to North America 

 and the Old World embraced in the fauna of Massachusetts comprise 

 all thus distributed now known, except two or three very boreal ones. 

 The faunae of the two continents are really quite different, — not totally 

 so, as has been claimed, — though represented largely by genera and 

 families common to the two. These and the circumpolar species show 

 that a close relationship exists between them, the resemblance being, 

 in fact, far greater than between the faunae of Southern Mexico and 

 Canada. The difference between the fauna? of the subtropical and cold 

 temperate zones on either continent is many times greater than between 

 the faunae of the temperate and boreal regions of North America and 

 the same regions of the Old World.* 



But four species have been attributed to the States adjoining Massa- 



* The distribution of vegetable life in zones, differing from each other in general char- 

 acter and corresponding in their limitation with climatic or isothermal zones, and their 

 similar succession at different altitudes on mountain slopes and in different latitudes at 

 the ordinary level of the land, was partially very early recognized, but first full}' demon- 

 strated only half a century ago, by Baron Alexander von Humboldt. It was somewhat 

 later before it was clearly shown that the same law holds in respect to the distribution 

 of terrestrial animal life, which was done in 1845 by Professor Louis Agassiz, 1 and 

 somewhat later still Professor Dana disclosed its presence in the distribution of ma- 

 rine life, in his admirable essay on the geographical distribution of the crostacea. 2 

 Yet most recent writers who have given attention to the geographical distribution 

 of animals appear to have overlooked this grand fact, and hence have been led to 

 adopt a highly artificial division of the earth's surface in respect to its primary ontologi- 

 cal regions. While geographical botanists have so generally recognized the influence 

 of climate, and especially of temperature, in determining the limits of distribution of 

 plants in latitude and in altitude, zoologists, with only a few exceptions, have very 

 imperfectly appreciated these important influences upon the distribution of animals. 

 While the relation of the present distribution of life to the existing means of communi- 

 cation between the different bodies of land and to the earlier conditions in this respect 

 are of the highest importance in investigations of this kind, if this is the only element 

 taken into account, as is sometimes the case, climatic influences being for the time over- 



1 " Note sur la Distribution G^ographique des Animaux et de l'Homrae." Bulletin 

 de la Societe" des Sciences Naturelles de Neuchatel, Tom. I, 1845. See also, by the same 

 author, a paper on the " Geographical Distribution of Animals," in the Edinburgh New 

 Philosophical Journal, Vol. XLVI, 1850, pp. 1-25. Also his " Sketch of the Natural 

 Provinces of the Animal World and their Relation to the different Types of Man," in 

 Nott and Gliddon's Types of Mankind, 1854, p. lviii. 



2 U. S. Expl. Exped. Reports, Crustacea, Vol. II, 1852, pp. 1451-1500. 



