54 INTRODUCTORY EXPLANATIONS. 



As with the ascending zones, it may be remarked also 

 of the types, that no decided lines of separation can be 

 drawn between them. They may be said to pass gradual- 

 ly into each other ; because the distribution of some spe- 

 cies is of such an intermediate character as to render the 

 choice of type to express it either dubious or optional. 

 Many species assigned to the British type, by becoming 

 less plentiful towards the northern or southern extremities 

 of the island, will thus pass into the English or Scottish 

 types. In like manner, those of the English type pass in- 

 to the Germanic or Atlantic. And between the Highland 

 and Scottish types the distinction is occasionally very 

 slight indeed. Still, in a general sense, these geographic 

 types do represent the realities of nature, thrown into 

 combinations according to some common points of simi- 

 larity. But it may also be allowed that the same facts 

 might be combined in other modes, and without much 

 greater departure from strictness in assigning some of the 

 species to such other modes of combination. Almost all 

 our native species, for instance, might be comprehended 

 in one of these two groups; namely, (1.) Species running 

 out northward, — (2.) Species running out southward. 

 Equally so, might they be associated into two other 

 groups; namely, (1.) Species running out eastward, — (2.) 

 Species running out westward. Some very general, and 

 some very local species would form the exceptions in ei- 

 ther of these two cases ; or, they would constitute an in- 

 termediate type, in which their northern or southern, their 

 eastern or western, tendency is either uncertain or non- 

 existent, at least, while our single island is alone under 

 consideration. On the contrary, it would likewise be pos- 

 sible to subdivide the six types into many others of a 

 minor or subordinate character. But on the whole, these 



