INBRED AND HYBRID ANIMALS AND THEIR VALUE 339 



studies of the factors affecting these characters; for the amount of variation 

 in random bred stocks was too small for practical analysis. 



This effect of inbreeding will undoubtedly be used more frequently when 

 it becomes widely known. Thus, embryologists derive much of their 

 information about normal processes of development from a study of abnor- 

 mahties, both naturally occurring and experimentally induced. They are 

 already using mutant types and will doubtless appreciate the value of an 

 inbred strain that provides abnormalities covering a wide range. 



Uniformity in time. — The genetic make-up of a random bred stock of 

 limited size will drift considerably from generation to generation. There- 

 fore, the characteristics of the stock may differ markedly at different times 

 and thereby cause trouble in a long-time investigation. The genetic con- 

 stitution of an isogenic inbred line can change only by mutation. The 

 phenotypic nature of the line is, therefore, less likely to vary with time. 

 This applies even when the phenotypic variation is greater in the inbred 

 strain. 



Combination of effects. — ]\Iore than one of the above effects can, of 

 course, often be used in a single investigation. Thus, a strain combining 

 high susceptibility to a bacterial infection with low variation in response 

 to inoculation may be used in a successfully uniform series of tests over a 

 long period. 



Differences between inbred lines. — It is to be expected that almost any 

 character studied will be shown to differ in different lines. Differences 

 have already been observed in a great many characters. They cover the 

 range from gene to behavior pattern, including countless biochemical, 

 cytological, histological and gross anatomical characters, and numerous 

 immunological, physiological and embr^'ological processes. We may 

 mention reported differences in calcium content of bones, chiasma frequency 

 in spermatocytes, histology of the adrenal, shape of the xiphoid process, sus- 

 ceptibility to yellow fever, ox}'gen consumption of excised tissues, develop- 

 ment of the mammary gland, and behavior response to a foreign male, as a 

 mere suggestion of the diversity. 



The value of inbred lines exhibiting these differences has been shown 

 in many ways. Often they have been used to demonstrate the importance 

 of hereditary factors, or, by crossing strains, to analyse the genetic differ- 

 ences. ]More frequently they have been employed in studying intermediate 

 causes of a difference, or in searching for associated phenomena to which 

 the difference might be attributed. Thus, Fekete (2) has investigated the 

 histology of the mammary glands in "high" and ''low" tumor strains of 



