172 THE MUTATION THEORY 



of which his method is unsuited. The whole literature of experi- 

 mental inquiry may be ransacked and not a single observation 

 on the smaller variations will be found. " For almost always 

 the points which tell are too fine to be dealt with in our analysis." l 

 Probably domesticated poultry, owing to the number and diversity 

 of the varieties and the ease with which they may be bred, and 

 the offspring compared with the parents and with one another, 

 have been the subject of more exhaustive experimental study than 

 any other type. The method of study in this case, as in all others, 

 has always been to cross the more dissimilar varieties a rumpless 

 fowl with one possessing a rump, a long-tailed fowl with a short- 

 tailed one, a white individual with a black, and so on. The 

 following is a fairly complete list of the characters studied : 



Comb form. Uropygium. Ear-lobe colour. 



Nostril form. Tail length. General plumage 



colour. 



Cerebral hernia. Vulture hock. Colour of hackles. 



Crest. Booting. Wing bars. 



Muff. Extra toe. Shafting. 



Beard. Colour of mandible and Body lacing. 



foot. 



Frizzling. Iris colour. Pencilling. 2 



Obviously we do not study the smaller variations, the ' fluctuations,' 

 when we cross any one of these characters with its opposite. 

 They are great differences which have arisen as sports or been 

 established by generations of careful selection, not those small 

 variations which commonly distinguish mating individuals when 

 the breeding is intra- varietal. 



289. Our power of observing small differences is proportionate 

 to our familiarity with the object of study. Thus the shepherd is 

 able to distinguish between the individuals of his flock and even 

 to note a good deal when he sees a strange flock ; but the ordinary 

 observer can hardly distinguish one variety of sheep from another 

 unless they display some glaring differences of size, shape, or colour. 

 To the Englishman landing in China the natives seem as much 

 alike as peas as in fact they are. He becomes trained in time to 

 distinguish Chinamen, but not peas. But from earliest infancy we 

 are forced to observe very closely the human beings among whom 

 we are reared. Indeed it is a main business of our lives to do so. 



1 Bateson, Mendel's Principles of Heredity, p. vii. 



2 Quoted from Inheritance in Poultry, by Mr C. B. Davenport, p. 81. 



