The conviction which has been gained by zoologists and botanists, that the British Islands once formed part of the Continent, is based on the present British fauna and flora. The remains, however, of animals which used formerly to live in these countries, such as the Mammoth, the Irish Elk, the Cave Bear, and many others, tell us the same tale. They could not have peopled England by swimming across the Channel, or even by walking across solid ice, as has once been suggested. Nothing but a land-connection induced them to explore this country more closely, and finally to decide on settling there. The origin of the British fauna will be discussed more in detail in the third chapter. The methods of investigation adopted, along with a general scheme of this book, will be found in the next. The manner in which the origin of the fauna of any particular continental area can be traced is very similar to that adopted in the case of an island. Portions of the continent of Europe can be shown to have been islands in former times. Thus the Crimea, now a peninsula united to the mainland by the narrow isthmus of Perekop, must have been an island in comparatively recent times. The absence of a number of striking and familiar South Russian species of mammals and reptiles proves this to have been the case. It was probably long after the appearance of man, though before historic times, that these changes took place. We shall learn in the subsequent chapters, that by a careful study of the fauna and flora the fact can be established, not only of the former connection of an island with a continent, but also whether such union existed (geologically speaking) within recent or more remote times. The better the fauna is known, both recent and fossil, the more precisely can the period of connection be indicated, and its duration determined. FOOTNOTES: [1] The numbers in brackets throughout this work refer to the page-number in the Bibliography at the end. [2] A map giving its exact distribution in Ireland will be found on p. 300, and a figure of the slug on p. 298. [3] The term _endemic_ will be employed throughout this work as applied to species peculiar to a country and not found elsewhere. _Autochthonous_ will be used in speaking of a species which has originated in a country to which, however, it is not peculiar; _e.g._, the Chamois is an autochthonous Alpine species, but occurs also in the Pyrenees and Caucasus. An _indigenous_ species is one native to a country, as opposed to the term "introduced," and is applicable to all species which have reached it by ordinary migration. [4] I might refer any one more specially interested in these introductions to an article on this subject in the _Irish Naturalist_ of March 1898, by Mr. Barrett-Hamilton. [5] I should recommend those who are particularly interested in the full history of the Irish frog to read the notes on this subject contained in vol. ii. of the _Irish Naturalist_. CHAPTER II.