These examples will illustrate the kind of questions it is the object of the present work to deal with. Every continent, every country, and every island on the globe, offers similar problems of greater or less complexity and interest, and the time has now arrived when their solution can be attempted with some prospect of success. Many {7} years study of this class of subjects has convinced me that there is no short and easy method of dealing with them; because they are, in their very nature, the visible outcome and residual product of the whole past history of the earth. If we take the organic productions of a small island, or of any very limited tract of country, such as a moderate-sized country parish, we have, in their relations and affinities--in the fact that they are _there_ and others are _not_ there, a problem which involves all the migrations of these species and their ancestral forms--all the vicissitudes of climate and all the changes of sea and land which have affected those migrations--the whole series of actions and reactions which have determined the preservation of some forms and the extinction of others,--in fact the whole history of the earth, inorganic and organic, throughout a large portion of geological time. We shall perhaps better exhibit the scope and complexity of the subject, and show 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.