HISTORY AND THEORIES OF EVOLUTION 391 



stationary while others advance." The explanation most 

 favored "ascribes the control of these modifications to the direct 

 effects of physiochemical factors on organisms." Professor 

 Whitman says, "Natural selection, orthogenesis, and mutation 

 appear to present fundamental contradictions, but I believe 

 that each stands for truth, and reconciliation is not distant." 



Mendel's law affirms that when mating takes place between 

 two animals unlike in some characteristic, the offspring will 

 often exhibit the characteristic of only one parent. This 

 characteristic is said to be dominant, while the character which 

 does not appear in the immediate descendants is said to be 

 recessive. The hybrids which result from the crossing of ani- 

 mals will produce a number of germ cells which bear only the 

 pure character of one parent and the same number which bear 

 only the pure character of the other parent. 



From this law follows the occurrence in the next and succeed- 

 ing hybrid generations of a definite number of forms in definite 

 numerical proportions. Thus, when gray rabbits are crossed 

 with albino rabbits, all the immediate offspring are gray, while 

 in the next generation produced by the breeding together of 

 these gray hybrids there will occur in nearly every case three 

 gray young to one albino. This is explained in the following 

 way : the second generation is all gray because in the zygote, or 

 fertilized germ cell, the chromosomes, or hereditary units, which 

 bear the gray character are more potent in the color of the young 

 than are the chromosomes bearing the albino character. This 

 gray character is said to be dominant ; the recessive albino char- 

 acters are not destroyed, but are carried over and give rise to 

 chromosomes of their own character, so that in the breeding 

 of two hybrids one albino germ cell from each of the two sexes 

 unite to produce one albino descendant. If one of the hybrid 

 dominants (gray) is mated with a recessive animal, half of the 

 young are hybrid dominants and half recessive. One who has 

 found out by experiment which are dominant and which are 

 recessive characters may produce several distinct types within 

 a species. If future experiments add support to this law, it will 

 then be explained how races suddenly spring into existence and 

 become established. 



By collecting evidence and arranging it in the form of pedi- 



