EXPERIMENTAL ZOOLOGY I93 



importance from the evolutionary standpoint. There is in- 

 creasing evidence, from experimental and statistical studies, 

 that there is another category of variation, which is the type 

 known as mutations or discontinuous variations ; that only 

 these are inherited, and hence only mutations are of import- 

 ance in evolution. The law of heredity discovered by Mendel 

 deals exclusively with inheritance of discontinuous variations 

 or mutations and it constitutes the first and only real law 

 of heredity which has ever been discovered. Its demonstration 

 and extension and modification constitute another triumph 

 for . the experimental method, though the conquest of 

 heredity has only begun. 



I can best present the fundamental facts of MendeHsm by 

 reference to the results obtained by the crossing of white and 

 gray mice. In the generation derived from this cross, no mat- 

 ter how many times repeated, the offspring are inevitably gray 

 and are indistinguishable from the type represented by the 

 gray parent, though obviously they cannot be identical with 

 the pure bred gray parent, since they have come from a gray 

 and a white. Though there are no whites among them, and, 

 therefore, no visible indication that one parent was of a pure 

 white breed, the capacity for whiteness is in them, since whites 

 appear in the next generation, when these hybrid grays are 

 bred among themselves. Before going further, two important 

 terms should be explained. The character which appears in 

 the first generation is known as the "dominant ;" the one which 

 does not appear is called the "recessive," and this relation be- 

 tween the members of a pair of characters is an important 

 phenomenon in Mendelism. In the first generation resulting 

 from this cross we find, therefore, one character, of the pair 

 under consideration, appearing to the exclusion of the other; 

 or, as we say, the one is dominant, the other is recessive. 



If now the individuals of this generation are bred among 

 themselves, a very different result is obtained; for we find in 

 the next generation that 75 per cent are gray and 25 per cent 



