136 LAND MAMMALS IN THE WESTERN HEMISPHERE 



before the publication of Darwin's book ; on the contrary, it 

 had attracted much interest as a study of facts, and this study 

 was one of the principal avenues by which Darwin approached 

 his great generalization. In his autobiographical fragment 

 he tells us : "I had been deeply impressed by discovering in 

 the Pampean formation great fossil animals covered with 

 armour like that on the existing armadillos ; secondly, by the 

 manner in which closely allied animals replace one another in 

 proceeding southward over the Continent ; and third, by the 

 South American character of most of the productions of the 

 Galapagos archipelago and more especially by the manner in 

 which they differ slightly in each island of the group." 



Obviously, before attempting to explain the facts of the 

 geographical distribution of mammals, we must first ascertain 

 what those facts are. The following brief sketch of the terms 

 used in describing geographical arrangement is summarized 

 from Mr. Wallace's ''Island Life." 



Though with fluctuating boundaries and subject to slow 

 secular changes, a mammalian species is limited to a fairly 

 definite area, which may be of immense or very restricted 

 extent, and throughout which it may be found in greater or 

 less abundance. Many species, however, are not distributed 

 continuously over the areas which they inhabit, but occur only 

 in suitable stations adapted to their habits and mode of life. 

 Thus, some will be found only where there are trees, others in 

 the neighbourhood of water, others only on open plains, etc. 

 A speciUc area is then the whole extent of country within which 

 the species may be found, while the stations are the limited 

 districts contained in the area which are exactly suited to the 

 habits of the species in question ; these stations may be hun- 

 dreds of miles apart, as in the case of mountain-tops, or they 

 may be close together. A marsh-living species, for example, 

 will occur in all the marshes throughout its area, whether these 

 be many or few, near together or widely scattered ; for such 

 a species marshes are its stations. 



