i 9 6 POPULAR SCIENCE MONTHLY. 



tinctness, and the two can be separated again at will. So it is in the 

 Belgian produced by cross-breeding with an albino. The albino char- 

 acter is there, though unseen, and will appear as a distinct entity when 

 the cross-bred reproduces, for it will be represented in approximately 

 half of the sex-cells formed by the cross-bred animal, the alternative 

 or Belgian character being represented in the other half. It is as if 

 the two pieces of glass, combined originally to illustrate the forma- 

 tion of a cross-bred animal, were separated again to illustrate the 

 formation of the reproductive elements by the cross-bred. For every 

 element formed having the opaque character, there will be another 



Fig. 4. A Guinea-pig with Long, Rough, Albino Coat. 



having the transparent character, but there will be no elements of an 

 intermediate nature. 



This simple principle, that in alternative inheritance sex-cells of 

 two sorts are formed by cross-bred individuals, constitutes one of the 

 most important discoveries ever made in the study of heredity. The 

 discovery was made about forty years ago by an Austrian monk, Gregor 

 Mendel, who was engaged in the study of cross-bred garden peas. It, 

 however, attracted little attention at the time and was soon forgotten. 

 Meanwhile, a great body of workers was studying with great minute- 

 ness the material basis of heredity, the sexual elements. Their investi- 

 gations disclosed in the cell a complete basis for just this kind of alter- 

 native inheritance and led up to the rediscovery of Mendel's law simul- 

 taneously by several different experimental breeders, foremost among 

 whom was the Dutch botanist, de Tries. 



