220 BEAN. 



come highly specialized, losing some qualities and gaining others by 

 a process of elimination and accretion, the crossing of types containing 

 such highly specialized characters may unite apparently dormant qual- 

 ities in each type and produce a new tyj^e unlike either of the original, 

 this being called a mutation, a sport of reversion (discontinuous varia- 

 tion). Heredity then is like producing like, with modifications, or 

 simply descent with variability. 



Heredity has three factors, a determinant, a modifier, and a law of 

 chance, and these three regulate variation, or heredity acting through 

 environment. 



The determinant, or the germ plasm, which has direct descent from 

 the germ cell of one generation to that of the next, has its activities 

 expounded by Weissmann, Galton, and Brooks, under the designation 

 of Weissmann's theory of heredity. 



The modifier has been variously set forth as selection by Darwin, use 

 and disuse by Lamarck, strength of parts by Eoux, organic selection by 

 Mark Baldwin, Osborne and Lloyd Morgan, and isolation and other 

 factors by many other biologists. 



The law of chance is Mendel's law, or- that of alternate variation, as 

 elaborated by Castle, Bateson and others and exemplified by the inherit- 

 ance of male from female and vice versa, through the specificity of the 

 accessory chromosome, as identified by Henking, McClung and Wilson, as 

 well as by certain hereditary afEections such as color blindness, peroneal 

 atrophy, congenital cataract, hemophilia, alcaptonuria and other abnor- 

 mal characters, like two joint digits, web fingers, etc. These usually ■ 

 show either dominant or recessive characteristics and are as a rule 

 inherited by the male through the female, the latter not being affected. 



The law of chance is the law by which the determinant acts, and 

 environment is the modifier. 



Mutation, or fluctuating variability, may be explained as the action 

 of the modifier on the determinant by accumulated and oft repeated 

 variation in a definite direction (determinate variation), as represented 

 by the paleontologic records of the horse;, and then the unexpected 

 crossing of long separated and specialized germ plasm or individual 

 characters, gives rise to reversions, sports, or mutations. Continued 

 intercrossing of differentiated characters, or specialized germ plasm, 

 causes fluctuating variability. Determinate variation, fluctuating varia- 

 bility and mutation are processes illustrating the modifications of the 

 determinate factor. 



Mendelism represents the action of the two extremes of a single char- 

 acter when crossed, such as black and white hair, long and short hair, 

 smooth and tufted hair, etc. In the first generation of such a cross the 

 dominant factor obscures the recessive and all appear to be dominant, 

 but in the next generation of a cross inter se the recessive factor reap- 

 pears, and always in a definite proportion. Wlien black and white 



