FERTILISATION 203 



to alter any single organ or structure without aftecting, in some 

 slight degree at any rate, all, or nearly all, the other parts of the 

 body. It may be argued, therefore, in criticism of the Mendelian 

 conception of unit characters, that it takes little or dio account of 

 the metabolism of the organism as a whole. Thus it has been shown 

 that in the case of presence or absence of hair pigment (which has 

 been regarded as a simple example of alternate characters, such 

 as can be superimposed experimentally upon other characters in 

 the course of two generations), there is a pronounced correlation 

 between albinism and other characteristics of the body, these 

 characteristics depending for their existence upon a common 

 metabolism. Moreover, the difficulty experienced by Wood 1 in 

 superimposing the complete hornlessness of Suffolk sheep upon 

 the white face of the Dorset horns, is probably another example 

 of the physiological correlation subsisting between different, and 

 apparently unconnected, structures. Originally, this case was 

 regarded as one of simple superposition, and Bateson 2 describes 

 the hornless character as having been transmitted "pure," but 

 subsequently many of the so-called hornless sheep were found to 

 have grown scurs. The explanation which I tentatively suggest is, 

 that the character of pure hornlessness was somehow or other 

 incompatible with the pure white-faced character, these two 

 characters being ordinarily indications of two sorts of metabolism, 

 in just the same kind of way as the beef -producing quality and 

 the milk-producing quality seem to be to some extent incompatible 

 in cattle. I am inclined to go further, and to suspect that many 

 of the other Mendelian cases, when examined more critically, will 

 show that no one character can be superimposed upon another, in 

 experimental breeding, without altering, though perhaps only very 

 slightly, the character upon which it has been superimposed. 



Experiments in which Merino rams were crossed with Shrop- 

 shire ewes 3 have shown that the more important characteristics 

 of the body or carcass (that is to say, the " mutton points " ) may 

 be transmitted to the third generation so as to reappear in new 

 combinations in the cross-bred sheep. Thus taking the four points, 

 " over the shoulder," " behind the shoulder," " top of leg," and " loin," 

 which are widely different in Merinos and Shropshires (being 



1 Wood (T. B.), " The Inheritance of Horns and Face-Colour in Sheep," Jour. 

 Agric. Science, vol. iii., 1909. 



2 Bateson, Mendel's Principles of Heredity, Cambridge, 1909. No doubt, 

 however, it is arguable that the scurs themselves represent unit characters, and 

 that if the scurs are of different kinds, these also represent unit characters 

 (which have hitherto somehow remained " latent "), and that if they occur with 

 different degrees of development, these again are unit characters. And so on. 



3 Mackenzie and Mai-shall, "The Inheritance of Mutton Points in Sheep," 

 Trans. Highland and Agric. ,S'o<'., 1917. 



