chap, xxiv.] SUMMARY AND CONCLUSION. 
501 
causes of the distribution of organisms, we will briefly indicate 
the scope and object of the several chapters, by means of which 
this general conception has been carried out. 
Beginning with simple and familiar facts relating to British 
and European quadrupeds and birds, I have defined and shown 
the exact character of “areas of distribution,” as applied to 
species, genera, and families, and have illustrated the subject 
by maps showing the peculiarities of distribution of some well- 
known groups of birds. Taking then our British mammals and 
land-birds, I follow them over the whole area they inhabit, and 
thus obtain a foundation for the establishment of “ zoological 
regions,” and a clear insight into their character as distinct 
from the usual geographical divisions of the globe. 
The facts thus far established are then shown to be necessary 
results of the “ law of evolution.” The nature and amount of 
“variation” is exhibited by a number of carious examples; 
the origin, growth, and decay of species and genera are traced, 
and all the interesting phenomena of isolated groups and dis- 
continuous generic and specific areas are shown to follow as 
logical consequences. 
The next subject investigated is the means by which the 
various groups of animals are enabled to overcome the natural 
barriers which often seem to limit them to very restricted areas, 
how far those barriers are themselves liable to be altered or 
abolished, and what is the exact nature and amount of the 
changes of sea and land which our earth has undergone in 
past times. This latter part of the inquiry is shown to be 
the most important as it is the most fundamental; and as it 
is still a subject of controversy, and many erroneous views pre- 
vail in regard to it, it is discussed at some length. Several 
distinct classes of evidence are adduced to prove that the grand 
features of our globe — the position of the great oceans and 
the chief land-areas — have remained, on the whole, unchanged 
throughout geological time. Our continents are shown to be 
built up mainly of “ shore-deposits ; ” and even the chalk, which 
is so often said to be the exact equivalent of the “ globigerina- 
ooze ” now forming in mid-Atlantic, is shown to be a com- 
paratively shallow-water deposit formed in inland seas, or in 
