34 THE ZOOLOGIST. 
climatal changes, the causes of which Mr. Wallace investigates 
at some length, with the aid of geologists, physicists and 
explorers. 
In the first half of his work (pp. 1—229), Mr. Wallace deals 
with “The Dispersal of Organisms,” its phenomena, laws, and 
causes. Beginning with simple and familiar facts relating to 
British and European quadrupeds, he defines the character of 
“areas of distribution” (Chap. II.) as applied to species, genera, 
and families, and illustrates the subject by maps showing the 
peculiarities of distribution of some well-known groups of birds. 
Taking our British mammals and land-birds, he follows them 
over the area they have been found to inhabit, and, classifying 
the facts of distribution (Chap. III.), obtains a foundation for 
the establishment of ‘zoological regions,” which are clearly 
characterized as distinct from the usual geographical divisions 
of the globe. 
The facts thus far established are then shown (Chap. IV.) to 
be necessary results of the ‘law of evolution.” The nature and 
amount of “variation” is exhibited by a number of curious 
examples; the origin, growth, and decay of species and genera 
are traced, and all the interesting phenomena of isolated groups 
and discontinuous generic and specific areas are shown to follow 
as logical consequences. 
Mr. Wallace next investigates (Chap. V.) the means by which 
animals are enabled to overcome the natural barriers which often 
seem to confine them to very restricted areas, the extent to which 
these barriers are liable to be altered or removed, and the nature 
of the changes of sea and land which have taken place in past 
times. The last-mentioned portion of the enquiry is shown to 
be the most important, as it is the most fundamental, and is 
discussed at some length, evidence being adduced to prove that 
the main features of our globe—the position of the great ocean 
and the chief land-areas—have remained, on the whole, un- 
changed throughout geological time. The general stability of 
continents, however, has been accompanied by constant changes 
of form, and insular conditions have prevailed over every part in 
succession, and the effect of such changes on the distribution of 
organisms is pointed out. 
In the succeeding three chapters (VI., VII. and VIII.) Mr. 
Wallace investigates very fully the question of geological climates 
