136 THE MIGRATION OF BRITISH BIRDS 



We will now proceed to analyze a little more closely 

 the distribution of birds within the limits of the British 

 Archipelago, and to ascertain whether the same laws, 

 the same general conditions of dispersal, have similarly 

 exerted their influence on the range of species in our 

 area, as we have already learned they have governed 

 the distribution of birds elsewhere. We have already 

 seen that the ultimate isolation of the British Islands 

 from continental land has had a marked effect upon the 

 distribution of terrestrial animals ; and in like manner 

 the isolation of Ireland from the greater land mass of 

 England and Scotland is significantly indicated by a 

 similar disparity in its mammalian and reptilian fauna. 

 I have in the present volume repeatedly insisted upon 

 the aversion displayed by birds to extend their emigra- 

 tions, or to increase their range across wide water 

 areas. Instances innumerable in all parts of the world 

 might be given in support of this assertion, but for 

 the present purpose we will confine our confirmatory 

 evidence to the species composing our own avifauna 

 alone. 



As I have already attempted to show, the land mass 

 of Ireland became isolated from England before Eng- 

 land itself became detached from continental land. 

 There is some evidence to suggest that the north-east 

 of Ireland continued to be joined to South-west Scot- 

 land for a much longer period, but as I hope presently 

 to demonstrate, this connection was of no service in 

 assisting the emigration of birds or animals to the Irish 

 area, although it may, and probably did, help them on 

 their emigrations northwards. The Law propounded at 

 the beginning of the previous chapter emphatically 



