388 THE BIOLOGY OF BIRDS 



Unit-characters and their Factors in the Inheritance. — 

 One of the great modern discoveries is the proof that an 

 inheritance consists, in part at least, of well-defined, relatively 

 stable, hereditary items. These have received various 

 names — factors, genes, or determinants. They are differ- 

 entiations in the germ-plasm corresponding to well-defined, 

 clear-cut characters in the offspring which are called " unit 

 characters." These are features which do not usually blend 

 or intergrade, which are either there intactly or not at all, 

 which are independently heritable. A particular colour of 

 iris, a particular pigment in the plumage, a crest on the head, 

 a spur on the foot, an extra toe, a great length of tail feather, 

 may be a unit character, which has its correspondent in the 

 germ-cell in a particular " factor " or " gene." It may be, 

 however, that several factors combine in the production of a 

 character {e.g. coloration), or that one factor has to do with 

 the development of several characters, just as the same 

 disease-taint may find expression in several difl^erent organs. 

 A " character " in a full-grown organism is some well- 

 defined, measurable feature, such as the colour of the iris, 

 the nature of the comb, the presence of a crest on the head, 

 the number of quills, the nature of the stomach, the size 

 of the keel on the breastbone, the number of toes, and so on. 

 In the strict sense the inheritance consists of the " factors " 

 or ** genes " corresponding to these characters, and not of 

 the characters themselves. It is necessary to insist on this 

 (though it may be pedantic to reiterate the distinction), 

 for one factor may affect several characters, and one character 

 may imply the co-operation of several factors. But, more 

 than this, the fully formed character is always the product of 

 the inherited " nature " and the appropriate " nurture." 

 The expression of a " factor " in development depends in 

 some measure on the fullness of the " nurture " — meaning 

 by this technical term of Galton's all manner of influences in 

 environment, nutrition, use and disuse. PecuHarities in 

 " nurture " often involve peculiarities in the development of 

 a character ; on the other hand, some peculiarity in a 

 character may be due to some novelty in the " factors." 



