II] Structural Characters : Anijuals 37 



Canaries. 



61. Crest is dominant to plain-head, as the non-crested 

 condition is called by fanciers (R.E.C. 19, p. 131 ; Daven- 

 port, 105). 



The type of crest which fanciers admire consists of feathers neatly 

 laid down over the head. To produce such birds crested individuals are 

 bred with plain-heads^ and it is clear that the exhibition type of crest is 

 a heterozygous form. When crested birds are bred together it is said 

 that an ugly, standing crest frequently is produced, and presumably this 

 is the homozygous type of crest. The mating of two crested parents is 

 by several authors said to give rise to some bald birds. Other writers 

 {e.g. Blakston) have ridiculed this statement, and formerly I was inclined 

 to regard it as a mere exaggeration, but Davenport in his recent paper 

 mentions bald heads as sometimes occurring among his crested birds. 

 He has kindly supplemented his published account with the statement 

 that the bald patch is an area "on the back of the head varying from four 

 to six millimetres in diameter practically without feathers and remaining 

 featherless throughout life. The crest, however, on top of the skull remains 

 perfectly evident, and often baldness can only be detected by blowing the 

 feathers." In no case was such a bald patch found in a plain-head. 



The bald patch on the occiput is recognized by Blakston ( Cage Birds, 

 p. 104) as a property of crested birds, and presumably the "balds" alleged 

 to come from the mating of two crests are birds homozygous for crest- 

 factor, in which the crest stands up and allows the bald patch to be seen. 

 Davenport had a crested bird without any bare patch, and he found that 

 the feathering in this region was due to a separate dominant factor. 



Animals and Plants in which Colottr-Characters have been 

 shown to have a Mendelian Inheritance. 



The phenomena of colour-inheritance are complicated 

 in several ways. Some of these complications which are 

 of great importance and interest will be considered in 

 subsequent chapters. It is, however, convenient to enu- 

 merate the genera in which Mendelian heredity has been 

 observed in order to illustrate the scope of the principle. 

 The followino- list of ^renera contains the chief of those in 

 which heredity according to a Mendelian system has 

 been shown to occur. In some of them as the result of 

 extensive research many Mendelian features of colour have 

 been discovered, and the existence of numerous colour- 

 factors is demonstrated. In others only one such factor 

 for colour has been detected. 



