INTRODUCTION. 5 



fertile among themselves, and the crosses (mongrels) between 

 them are also fertile, though there are exceptions to this among 

 plants. 



The old view as to the " origin of species " was that they were 

 all separately created, but it is now almost universally held by 

 scientists that living species have been evolved from pre-existing 

 ones, and that varieties are to be regarded as species in process 

 of evolution. A brief sketch of the evolution theory will be 

 given in another place. 



(5) Distribution. Rana temporaria is a very widespread species, 

 ranging over the greater part of Europe, N. Africa, N. Asia to 

 Japan, and N. America. It is absent, however, from Iceland and 

 N. Scandinavia. 



We have less definite knowledge as to the range in time of 

 this species, but in this country the bones of a frog closely 

 resembling it have been found in a deposit * accumulated at a time 

 when the British fauna included species of Elephant, Hippopo- 

 tamus, Rhinoceros, and Hyaena. 



(6) Phylogeny (^Etiology) makes use of data supplied by all 

 the other departments to work out the evolution of groups. The 

 Amphibia must be regarded as one branch of a huge genealogical 

 tree, its orders corresponding to subdivisions of this branch, its 

 families and genera to still smaller ramifications, and its species 

 to the ultimate twigs. 



2. Differences between Animals and Plants. No difficulty is 

 experienced in distinguishing a higher animal from a higher 

 plant. A Frog, for instance, differs in the compactness of its 

 form from, say, a tree, and this difference is correlated with the 

 nature of the food. A tree spreads out leaves for the absorption 

 of carbon dioxide, while it takes up by its roots water charged 

 with inorganic salts. Its food is of simple kind and gaseous or 

 liquid in nature ; hence the diffused branching form which offers 

 a large external absorptive surface. A Frog, on the other hand, 

 requires much more complex food, part of which must be albu- 

 minous in nature, and it can only get this by devouring other 

 organisms, in this case insects; while tadpoles chiefly live on 

 plants, upon which, indeed, all animals directly or indirectly 

 depend. This complex organic food being solid the Frog requires 

 an internal digestive cavity for its reception, and there is an obvious 

 * The Cromer Forest-Bed. 



