8 
ISLAND LIFE. 
[rART I. 
and in many cases it can 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 interpreta- 
tion 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 . 1 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 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 
epochs, and thus enables us to prove that certain areas have been 
long isolated and the fauna and flora allowed time for special 
development. Here, too, our knowledge is exceedingly im- 
perfect, though the blanks upon the geological map of the world 
1 The general facts of Palasontology, as bearing on the migrations of 
animal groups, are summarised in my Geographical Distribution of A nimals, 
Vol. I. Chapters VI., VII., and VIII. 
