32 The Oologisis' Record, June 1, 1923. 



manner in which sub-species are evolved. At present naturalists 

 are, roughly, divided into two schools — the one claiming that all 

 evolution is due to environment, the second that all specific change 

 is due to mutation. Both are probably right, and the difference 

 is one of words rather than of facts. To me it seems clear that 

 environment is the cause of such evolution as is necessary to fit a 

 bird to its surroundings. That is to say, it intensifies or reduces 

 colour, increases or decreases length of limb, wing, bill, etc., and 

 generally performs the work of elimination of all adverse characters, 

 whilst it perpetuates and perfects all such as are beneficial. At 

 the same time, for the initiation of new characters we have to look 

 to mutation. 



But even when differences are initiated by mutation, whether 

 accidental or inherent, the question as to whether these shall persist 

 or die out is settled by environment. There are, roughly speaking, 

 three forms of mutation — beneficial, harmless and harmful. Now, 

 if through mutation a bird is provided with a character M^hich will 

 be of assistance to it either in competing with other individuals of 

 its own species or in rendering it more or less immune to some 

 destructive influence, whether this be vermin, heat, cold, wet or 

 any other factor, then the individuals which inherit this feature 

 will persist in larger comparative numbers than those which do 

 not have it. If, on the other hand, the mutational feature is 

 harmful, we shall find it very speedily eliminated, for it will never 

 be allowed to attain fixity, the individuals possessing it being 

 destroyed at a greater rate than those without it. 



This leaves only the third kind of mutation, i.e. the harmless. 

 Man}^ of these mutations are merely the abnormal expression of 

 vigour and they persist because the more normal manifestations of 

 vigour, such as increase in size, speed of wing, access of voice, etc., 

 would be prejudicial to the continuation of the race ; whereas other 

 features, such as crests, breast plumes, brilliancy of colouring, 

 etc., are not injurious in this particular instance. 



Accidental variations which are not accompanied by imusual 

 vigour are not likely ever to become permanent unless they come 

 under the first class of mutation, the beneficial. On the other 

 hand, when the new character is an expression of energy and vigour, 

 the individual bird possessing it will have more chance of handing 

 it on to his progeny, and will also probably have a more numerous 

 progeny to hand it on to than will other birds possessing less vigour. 



