The Oologists' Record, June 1, 1923. 35 



In island forms our breaks are naturally more and our connec- 

 tions less definite between various sub-species than they are in 

 continental forms, but, as I have already said, in these we must 

 look for individual variation to show us the connecting links. 



With many naturalists " isolation " is insisted on as the dominant 

 factor in the creation of species but, from this point of view, it is 

 quite impossible to define isolation. It may be complete, semi- 

 complete or very incomplete. The cause of isolation may be an 

 actual gap, narrow or wide, in land boundaries as in islands and 

 continents ; it may be an almost equally isolating barrier such as 

 a snow range, desert, etc., or it may be only a partial barrier, 

 such as a big river or an area of country less suitable to the occupant 

 than those on either side of it. 



When a species becomes so dominant that it overcomes other 

 destructive elements, it so increases in number that its tendency is 

 to increase its area also. The consequences are three : first, the 

 area invaded is so unsuitable that the species dies out at once ; 

 secondly, it is sufficiently suitable provided certain modifications 

 are obtained ; or, thirdly, the species carries on a precarious exist- 

 ence and in the struggle for existence continues its wanderings into 

 more suitable countries. In these latter it once more increases and 

 multiplies under conditions which perpetuate it, either in its original 

 form or with some modifications which make it a new sub-species. 



Often, however, it must happen that the birds which come under 

 the third category eventually disappear entirety in the intermediate 

 and less sidtable countries, so that we then have a complete break 

 in continuity of geographical range, the intermediate forms are 

 wiped out of existence, and the sub-species, having lost all connecting 

 links with its parent stock, becomes a full species. 



Isolation and continuous in-breeding will not create new species 

 or sub-species unless the environment evolves some new character 

 or encourages some character accidentally commenced ; on the 

 other hand, continuous in-breeding may cause constitutional 

 disturbances resulting in such freak characteristic as the dancing 

 of the dancing mouse. 



Even in cases similar to that of the dancing mouse or to the 

 never-ending new forms produced among domestic animals, it 

 must be remembered — a fact which seems to be constantl}^ forgotten 

 — that in these domestic animals environment never comes into 

 play. Their surroundings are purely artificial, and the man who 



