8 



ISLAND LIFE. 



[rART I. 



and in many cases it can never now be obtained owing to tlie 

 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.-^ 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 Pal £E ontology, as bearing on the migrations of 

 animal groups, are summarised in my Geographical Distrihution of Animals, 

 Vol. I. Chapters VI., VII., and VIIL 



