MUSEUM OF COMPARATIVE ZOOLOGY. 247 



Table V, comprising those species that do not occur north of the 

 Alleghanian fauna, embraces but one of relatively large size, — Vul- 

 pes virginianus, — which is also the only carnivore ; the others are 

 two moles and four rodents. The presence of the species of this list, 

 and the absence of those of the preceding, form the faunal differences 

 that, among mammals, distinguish the Alleghanian from the Canadian 

 fauna. The other thirty-three species of land mammals represented 

 in the fauna of Massachusetts, and which are common to the other New 

 England States, New York, the northern tier of the States westward to 

 the Mississippi, and the greater portion of the Canadas, range widely both 

 to the north and to the south, and some of them also to the westward, 

 extending throughout the colder parts of the northern hemisphere, as is 

 indicated by Tables VII and VIII * 



* In this connection a word in reference to the nature of faunae may not be out of 

 place, since naturalists of some eminence, but who cannot have thoroughly investigated 

 the subject, appear to think that no faunal districts are recognizable unless there is an 

 entire or almost an entire change in the species represented, while some altogether dis- 

 card such distinctions. Such an extensive change more properly characterizes the 

 larger divisions in geographical zoology, as the provinces and realms., rather than faunae. 

 It rarely happens that any species is restricted within the limits of a single fauna, and 

 also rarely within those of two. There is not a single well-known species of mammal or 

 bird but inhabits (taking the breeding range only of the latter) an area embracing two 

 or more faunae, and but few that do not range over more than two. The greater part 

 extend over three, and a large proportion have a still wider distribution, as shown by 

 Tables VII and VIII (see remarks respecting these beyond). But in going north or 

 south from any point within the temperate zones, one observes at certain intervals (gen- 

 erally of about six or seven degrees of mean annual temperature) a marked change in 

 the species, through the disappearance of some and the appearance of others; this change 

 giving rise to well-marked differences in the general facies of the fauna at points not 

 far distant. The habitats of species being in the main nearly coincident in their northern 

 and southern boundaries with isothermal lines, and not with paralells of latitude; and 

 since a number of species usually disappear at nearly the point at which a number of 

 others first make their appearance, the limits of faunae are thus readily defined, at least 

 approximately. As isotherms necessarily vary with every inequality in the surface of 

 the country, they rarely correspond, as is well known, with the parallels of latitude ; 

 and plants and animals sharing the same apparent irregularity in their distribution, 

 some naturalists have been led to discredit the existence of recognizable zoological and 

 botanical districts, or of any definite system in the distribution of animals and plants. 



Faunre, then (the term fauna in its restricted sense being usually and properly em- 

 ployed to designate the smallest zoologieo-geographical district), it may be added, are 

 characterized by the peculiar association of species. Generally about twenty-five per 

 cent of those embraced in either of two adjacent faunae are absent from the other. 

 Rarely do adjoining faunae differ essentially in genera, though necessarily more or less occa- 

 sionally. The absence or presence of genera, sub-families, families, and even sometimes 

 orders, more properly characterizes the higher sub-divisions, as provinces and realms. 



