GROWTH AND MORPHOLOGICAL CHARACTERS. 53 



Physiological correlation is not always explained by gametic coupling. 

 It is not difficult to understand how a whole organism or parts of an 

 organism are permanently influenced by even normal conditions. For 

 example, we should hardly expect a normal but very small rabbit to 

 have as large ears as a large rabbit, although both might have theo- 

 retically the same set of genes for ear-size. In fact, if one carried out 

 the whole scheme of independent size-factors without reference to 

 physiological correlation it would lead to an absurdity. If a guinea- 

 pig had genes for a small radius and a large ulna, or for a large tibia 

 and a snaall fibula, would the animal be a cripple? In dealing with 

 the inheritance of size of certain bones of the body, one can not over- 

 look the influence which other parts, or even the whole body itself, 

 may have upon the development of particular characters studied, 

 irrespective of the hypothetical genes. 



It is well known that certain color characters in plants and animals 

 develop only through the interaction of two or more independently 

 transmitted factors. Thus, the factor for the agouti pattern in the 

 hair of rodents acts only when black or brown is present in the zygote; 

 but black or brown pigments in turn are restricted to the eyes and 

 extremities unless the extension factor is present. It may be added 

 that the basic color factor must also be present in order to activate 

 the development of color. Therefore, to obtain the agouti pattern it 

 is necessary to have at least four independently heritable factors, viz, 

 the color factor, the extension factor, the brown or black factors, and 

 the agouti factor. When we recall such facts as these, and realize 

 that several or many factors may interact in the production of size- 

 characters, we see how difficult it is to attempt or rather attain a 

 satisfactory solution. 



Considering briefly the evidence which tends to show that a number 

 of factors may exist for one and the same visible character, we find 

 comparatively few experiments. Most of these are in plants. Nillson- 

 Ehle (1909,1911) paved the way by showing how some apparently 

 continuous variations might be interpreted as discontinuous variations. 

 The black glumes of oats, he showed, might be due to two factors, 

 either of which alone could cause the development of black in the glume. 

 If a plant homozygous for both kinds of black (B1B1B2B2) was crossed 

 with a plant lacking black (bibib2b2), the heterozygotes were black 

 and hold a formula BibiB2b2. Crossing the heterozygotes inter se gave 

 an average of one entirely recessive individual in every 16. It proved 

 to be a simple dihybrid cross, in which 15 of every 16 r2 individuals 

 carried at least one dose of a dominant factor and were black. The 

 F2 generation should theoretically consist of 9 BiB2 : 3 Bib2 : 3 biB2 : 

 1 bib2. Since either factor Bi or B2 caused a development of black 

 in the glume, the first three classes were alike black. Subsequent 



