The geographical distribution of animals, with a study of the relations ...

54 DISTRIBUTION OF ANIMALS. [part i.and this, it may be said, proves their fundamental unity andthat they ought to form but one primary region. To obviatesome of these difficulties a binary or dichotomous division issometimes proposed; that portion of the earth which differsmost from the rest being cut off as a region equal in rankto all that remains, which is subjected again and again tothe same process.To decide these various points it seems advisable that convenience,intelligibility, and custom, should largely guide us.The first essential is, a broadly marked and easily rememberedset of regions ;which correspond, as nearly as truth to naturewill allow, with the distribution of the most important groupsof animals.What these groups are we shall presently explain.In determining the number, extent, and boundaries of theseregions, we must be guided by a variety of indications, sincethe application of fixed rules is impossible. They should evidentlybe of a moderate number, corresponding as far aspracticable with the great natural divisions of the globe markedout by nature, and which have always been recognized bygeographers. There should be some approximation to equalityof size, since there is reason to believe that a tolerably extensivearea has been an essential condition for the development ofmost animal forms ; and it is found that, other things beingequal, the numbers, variety and importance of the forms ofanimal and vegetable life, do bear some approximate relationto extent of area. Although the possession of peculiar familiesor genera isthe main character of a primary zoological region,yet the negative character of the absence of certain familiesor genera is of equal importance, when this absence does notmanifestly depend on unsuitability to the support of the group,and especially when there is now no physical barrier preventingtheir entrance.This will become evident when we consider thatthe importance of the possession of a group by one region dependson its absence from the adjoining regions ; and if there isnow no barrier to its entrance, we may be sure that there hasonce been one ;and that the possession of the area by a distinctand well balanced set of organisms, which must have been slowly