in Zoological and Botanical Geography. 13 



isolate most mammals and many reptiles, while insects 

 Lave still various means of passing it. 



Another consideration which must help to determine the 

 amount of specific peculiarity in a given region, is the 

 average rate at which specific forms have changed. Palae- 

 ontologists have determined that mammalia have changed 

 much more rapidly than moUusca, from the phenomena of 

 the comparatively recent extinction of so many species of 

 mammals, whose remains are found along with existing 

 species of shells. From the evidence of the distribution 

 of existing species, birds would appear to have changed at 

 least as quickly as mammals, and insects, in some cases, 

 perhaps more so ; owing, no doubt, to their very small dif- 

 fusibility, and the readiness with which they are aftected 

 by local conditions. 



Taking the various facts and arguments now brought for- 

 ward into consideration, it appears evident that no regions 

 (be they few or many in number) can be marked out, which 

 will accurately represent the phenomena of the geographical 

 distribution of all animals and plants. The distribution of 

 the several Classes, Orders, and even Families, will difi'er, 

 because they differ in their difiPusibility, their variability, 

 and their mode of acting and reacting on each other, and 

 on the external world. At the same time, though the 

 details of the distribution of the different groups may differ, 

 there will always be more or less general agreement in this 

 respect, because the great physical features of the earth — 

 those which have longest maintained themselves unchanged 

 — wide oceans, lofty mountains, extensive deserts — will 

 have forbidden the intermingling or migration of all groups 

 alike, during long periods of time. The great primary 

 divisions of the earth for purposes of natural history should, 

 therefore, correspond with the great permanent features of 

 the earth's surface — those that have undergone least change 

 in recent geological periods. Later and less important 

 changes will have led to discrepancies in the actual distri- 

 bution of the different groups ; but these very discrepancies 

 will enable us to interpret those changes, of which they 

 are the direct effects, and very often the only evidence. 



From this examination of the anomalies that occur in 



