ow that any intelligent study of it was almost impossible till quite
recently, if we concisely enumerate the great mass of facts and the number
of scientific theories or principles which are necessary for its
elucidation.

We require then in the first place an adequate knowledge of the fauna and
flora of the whole world, and even a detailed knowledge of many parts of
it, including the islands of more special interest and their adjacent
continents. This kind of knowledge is of very slow growth, and is still
very imperfect;[2] and in many cases it can {8} never now be obtained owing
to the reckless destruction of forests and with them of countless species
of plants and animals. In the next place we require a true and natural
classification of animals and plants, so that we may know their real
affinities; and it is only now that this is being generally arrived at. We
further have to make use of the theory of "descent with modification" as
the only possible key to the interpretation of the facts of distribution,
and this theory has only been generally accepted within the last twenty
years. It is evident that, so long as the belief in "special creations" of
each species prevailed, no explanation of the complex facts of distribution
_could_ be arrived at or even conceived; for if each species was created
where it is now found no further inquiry can take us beyond that fact, and
there is an end of the whole matter. Another important factor in our
interpretation of the phenomena of distribution, is a knowledge of the
extinct forms that have inhabited each country during the tertiary and
secondary periods of geology. New facts of this kind are daily coming to
light, but except as regards Europe, North America, and parts of India,
they are extremely scanty; and even in the best-known countries the record
itself is often very defective and fragmentary. Yet we have already
obtained remarkable evidence of the migrations of many animals and plants
in past ages, throwing an often unexpected light on the actual distribution
of many groups.[3] By this means alone can we obtain positive evidence of
the past migrations of organisms; and when, as too frequently is the case,
this is altogether wanting, we {9} have to trust to collateral evidence and
more or less probable hypothetical explanations. Hardly less valuable is
the evidence of stratigraphical geology; for this often shows us what parts
of a country have been submerged at certain epo