ZOO-GEOGRAPHICAL REGIONS. 
397 
The variable areas occupied by different species, which rightly 
understood are also an index to their relative antiquity, have led to 
the division of the earth’s surface into a number of regions or districts 
to express the differences shown by one area in comparison with 
another. These divisions, if correct, should be broadly applicable to 
other forms of life and confirm the unity of the plan of development, 
not only emphasizing the differences in character and structure of the 
inhabitants, but also indicating the relative ages of the groups 
characterizing the divisions, and as the areas represent marked periods 
in migration from the evolutionary centre, the organisms inhabiting and 
characterizing them will change with the progress of time, and in the 
future, as in the past, will support groups of animals and plants different 
from those constituting their predominant features at the present day. 
The Geographical Regions into which the globe may thus be 
broadly separated are known as the Arctic, Palsearctic, Nearctic, 
Ethiopian, Palseotropical, Neotropical, and Australasian, and these 
divisions, all of which may be still further sub-divided, often accord 
more or less closely with striking physical features either of the 
present day or of comparatively recent geological epochs, the more 
ancient and pronounced the barriers to dispersal the more striking 
the differences of the faunas of the districts they separate. 
To illustrate the special characteristics of the different geographical 
regions, I have again selected as most suitable the members of the 
family Helicidte, not only on account of their world-wide distribution, 
but because the general organization of the group has received such 
careful and thorough study at the hands of a number of competent 
observers, and their philosophical division by Pilsbry, according to 
the general character of their internal organization, assists ns to see 
how beautifully their gradations of structural complexity coincide 
with the broad features of their distribution, so that we are often 
able merely from a study of the relative remoteness or contiguity of 
their habitat to the evolutionary area to predicate with considerable 
confidence the degree of structural perfection to which they have 
attained, as the most highly organized groups occupy the actual 
evolutionary area, while the most simply constituted are furthest 
removed therefrom, showing that the various steps leading to this 
greater perfection of organization have all originated within the district 
occupied by the latest developed and morphologically higher groups, 
from whence they have each in turn spread over the globe, overcoming 
