320 OUTLINES OF E VOLUTION ABY BIOLOGY 



tit (Parus palustris), which has two areas of distribution separated 

 from one another by an interval of four thousand miles in 

 Europe and Asia Minor on the one hand and in Northern China 

 on the other. 



The size of the area over which a species may range varies 

 immensely, in some cases comprising an entire continent, or even 

 more, and in others only a few square miles. Thus the leopard 

 ranges over the whole of Africa and most of Southern Asia, while 

 the Tuatara (Fig. 113) is confined to certain small islands off the 

 coast of New Zealand r and certain species of humming birds 

 are said to occur only on the volcanic peak of Chimborazo in the 

 equatorial Andes * 



An area of generic distribution is the sum of the areas of 

 distribution of all the species which are comprised within the 

 genus, and thus genera have usually a much wider geographical 

 range than species. Families, again, have a wider range than 

 genera and orders than families, and so on with groups of still 

 higher value. In short, the more comprehensive the group the 

 larger will be its area of distribution, until we find that the sub- 

 kingdoms or phyla are cosmopolitan, ranging more or less over 

 the entire world, wherever a suitable habitat is to be found. 



The reason why species are rarely, if ever, cosmopolitan in 

 their distribution is that they are confined within their own 

 limited areas by the existence of physical conditions which con- 

 stitute what are called barriers to migration. Such barriers 

 either form absolutely insuperable obstacles to the passage of the 

 species in question or they may be surmounted only at rare 

 intervals and by some happy chance. 



The nature of the barriers varies, of course, with the species 

 concerned, and what is a barrier to one may be a high road 

 to another. In the case of marine animals the principal barriers 

 are continents and temperature conditions, while the deep sea 

 itself acts as a barrier to the distribution of shore-dwelling or 

 littoral forms. For terrestrial animals the chief barriers are 

 seas, rivers, mountain ranges, deserts and climate generally ; for 

 fresh-water animals land and sea. In the case of plants the 

 barriers to migration are very much the same. 



It may be laid down as a general law that every organism, 



whether animal or vegetable, at some period or other of its 



/ existence is specially adapted so as to secure dispersal, either by 



its own exertions or by the action of some external agency. By 



