INTRODUCTORY 29 



own. Were Plotheia frontalis constructed on a chemical plan 

 which admitted of no variation, the countless varieties would 

 not have been produced ; and if one of its varieties had an over- 

 whelming success out of all proportion to that of the rest, then 

 the species would soon become monomorphic again. We 

 cannot declare that Natural Selection has no part in the deter- 

 mination of fixity or variability; nevertheless looking at the whole 

 mass of fact which a study of the incidence of variation provides, 

 I incline to the view that the variability of polymorphic forms 

 should be regarded rather as a thing tolerated than as an element 

 contributing directly to their chances of life; and on the other 

 hand that the fixity of the monomorphic forms should be looked 

 upon not so much as a proof that Natural Selection controls 

 them with a greater stringency, but rather as evidence of a natural 

 and intrinsic stability of chemical constitution. 



Compare the condition of a variable form like the male 

 Ruff (or in a less degree the Red Grouse in both its sexes) with 

 that of the common Pheasant which is comparatively constant. 

 In the Pheasant no doubt variations do occur as in other wild 

 birds, but apart from the effects of mongrelisation the species 

 is unquestionably uniform. Could it seriously be proposed 

 that we should regard the constancy of the pheasant's plumage 

 in this country as depending on the special fitness of that type 

 of colouration? Even if the pheasant be not an alien in Western 

 Europe, it has certainly been protected for centuries, and for a 

 considerable period has existed in a state of semi-domestication. 

 Such conditions should give good opportunity for polymorphism 

 to be produced. In some coverts various aberrations do of 

 course occur and persist, yet there is nothing indicative of a 

 general relaxation of the fixity of the specific type, and the pheas- 

 ant remains substantially a fixed species. 1 The common pheasant 

 (Phasianus colchicus) even shows little of that disposition to 



1 Howard Saunders (Illust. Manual of British Birds, 1899, p. 499) states that 

 there is evidence that the pheasant had become naturalized in the south of England 

 before the Norman invasion. He adds, "little, if any, deviation from the typical 

 P. colchicus took place up to the end of last century, when the introduction of 

 the Chinese Ring-necked P. torquatus commenced, which has left almost indelible 

 marks, especially with regard to the characteristic white collar." 



