34 , The Oologists' Record, June 1, 1923. 



taking the place of environment and eliminating characters which 

 are not desired and protecting those which are. 



The procedure is simplicity itself when compared with Nature, 

 for all man has to do is to continue selecting for breeding purposes 

 those individuals which show to the greatest extent the freak 

 characteristic he desires to make permanent. 



In Nature there are a thousand-and-one influences, often con- 

 flicting, and the progress towards comparative permanency is 

 naturally more slow. Parallel instances of man's and Nature's 

 working are not hard to find. Take one of the simplest. Man 

 has produced white rabbits, mice, etc. ; so has Nature produced 

 white animals, or animals which are white in winter. Man has 

 produced his pink-eyed whic.e rats by constant selection for breeding 

 purposes of those rats which have possessed some constitutional 

 character resulting in loss of pigment. Nature has gradually 

 evolved, by the constant survival of the fittest, white animals 

 which are protected in their winter habitat by their want of colour 

 from their enemies of all kinds. In this category come such animals 

 as foxes, the weasel tribe, certain Finches, etc. 



What, then, is the difference between species and sub-species ? 

 To me the difference is this : Between species and species there 

 is some definite point at which intergradation ceases, and the bridge 

 is lacking which connects the one with the other ; in sub-species 

 there is a continued gradation which connects the two forms without 

 a definite break. The exact extent of the break necessary to 

 transform a sub-species into a species cannot be laid down, for it 

 varies in degree in different birds ; but if we add to our previous 

 formulae the rule that two sub-species cannot inhabit the same 

 breeding area, we at once advance very considerably. 



Take, for instance, Phylloscopus borealis. Here we have a 

 Warbler which divides, broadly, into a western and eastern race 

 differentiated by wing formulae and to a less extent by size. In 

 the winter both races may be shot off the same tree in certain 

 countries, but their breeding areas are apart, and in the intermediate 

 countries between their areas the wing formula is not fixed and 

 the size is also intermediate. But if we turn to some of the 

 Acrocephali we shall find that whilst a difference in wing formula 

 appears to be merely of sub-specific value in some species, as in 

 agricola, yet it is of specific value in others such as scirpaceits and 

 phragmitis. 



