376 BULLETIN OF THE 



"Wagner,* Dana,f and other?, subsequently establishing the same in re- 

 gard to animals ; the distribution of both plants and animals being prima- 

 rily determined by the same influences. It has been further shown that 

 these influences are mainly climatic, temperature having been justly 

 recognized as governing the limitation, especially in latitude, of not only 

 the species, but of faunae and flora;. Their limitation in longitude is 

 likewise as directly determined by climatic influences,]: though indirectly 

 by physical barriers, as oceans, mountain chains, and deserts. Humidity, 

 in many instances, is scarcely a less, and in some cases a more, power- 

 ful limiting agent than temperature, plants being highly sensitive to 

 hygrometric conditions, and their distribution intimately affects that of 

 animals, since the existence of the latter is dependent upon the presence 

 of the former, and their variety and numbers upon the degree of luxu- 

 riance of the vegetation. The faunal and floral zones hence coincide 

 in their limitation in latitude with the climatic zones, but by no means 

 necessarily with the geographical circles ; isothermal lines, and not paral- 

 lels of latitude, forming their boundaries. Their limits in longitude are 

 determined by the influence geographical barriers, especially long chains 

 of high mountains, exert upon climate. 



* Wagneb, Andreas. " Die geographische Verbreitung der Siiugethiere," Abhand- 

 lungen de: baierischen Akademie der Wissenschaften, Math. Phys. Classe, Band IV, 

 Abth. I, pp. 1-146, 2d Abth., pp. 1-108, 3d Abth., pp. 3-114. Mit 9 Karten, 1844- 

 1846. 



t U. S. Exploring Expedition Report, Crustacea, Vol. II, pp. 1451-1500, 1852. 



J I am aware of the diversity of opinions still prevalent among naturalists in regard 

 to the influence climate exerts in determining the geographical distribution of species, 

 and that many writers on this subject attribute to it only a slight importance, or alto- 

 gether ignore it. The limits of these preliminary remarks will not allow of an extended 

 comparison of the views of different authors on this point, nor a detailed consideration 

 of the supposed objections that have been raised against the proposition above expressed. 

 I agree with Mr. Andrew Murray in his remark, that, although "various authors have 

 endeavored to embody the differences between the faunas and floras of the different re- 

 gions of the globe into some kind of system, .... they, with one or two exceptions, 

 have worked upon no definite principle, and the result has been a mere catalogue of re- 

 gions which possessed peculiarities without distinguishing their relative importance, 

 or their relation to each other" (Geographical Distribution of Mammals, p. 296, 4to, 

 London, 1866),— a remark which unfortunately semis in some degree applicable to 

 Mr. Murray's own generalizations. That temperature is n powerful limiting influence 

 affecting the range of species, especially in respect to their northward and southward 

 extension, is so easily demonstrable that I am surprised to see it -till questioned. I have 

 myself subjected this principle to a rigid examination in studying the distribution of 

 the animals and plant- of Eastern North America, and have been surprised at the exact 

 coincidence I have almost constantly met with between their northern and southern 



