344 



The Journal of Heredity 



In reality alternative inheritance is 

 not limited to characters that show the 

 Mcndelian reactions, but extends into 

 other fields of heredity where the 

 theory of germinal segregation and 

 alternative transmission of unit char- 

 acters does not apply. The sudden 

 changes of characters often shown in 

 successive intemodes of the same plant 

 are as abrupt and as definite as any 

 mutative change or Mcndelian alterna- 

 tion, and yet these changes of charac- 

 ters in intemodes take place without any 

 opportunities for the segregation and 

 redistribution of gens in the formation 

 of new germ cells, as required by the 

 Mcndelian theory of alternative trans- 

 mission. Such changes of character, 

 occurring during the development of 

 the same individual plant or animal, 

 must represent alternative expression of 

 characters, instead of alternative trans- 

 mission. And after this distinction is 

 appreciated it appears reasonable to 

 expect that other sudden changes or 

 abrupt differences among the individuals 

 of the same stock may represent varia- 

 tions in the expression of characters, 

 rather than differences of transmission. 

 Changes of expression must be con- 

 sidered in the study of heredity, as well 

 as differences of transmission.* 



THE POLYGENIC INTREPRETATION. 



Instead of a single set of gens, as as- 

 sumed in the theory of Mcndelism, a 

 multiple transmission of many alterna- 

 ti\'e gens should l)e recognized, enough 

 to rei)resent the whole range of ancestral 

 diversities. When the variations of 

 domesticated stocks are compared with 

 the diversities of related wild types the 

 same general range of character is 

 found. Many of the so-called "new 

 characters" of mutative variations 

 should be considered rather as supj^res- 

 sions or reap])earances oi old characters. 

 The ])henomena of reversion and reca- 

 pitulation indicate that the transmission 

 of characters is entirely indei^cndent of 

 exjiression, and that characters may be 

 transmitted in latent form for manv 



generations and still retain their power 

 of returning to visible expression.* 



A polygenic interpretation of heredity 

 docs not conflict with the facts of 

 Mcndelism and mutation, and is in 

 better accord with other facts of biology, 

 both evolutionary and physiological. 

 From a polygenic viewpoint, mutations 

 and Mcndelism appear as differences 

 in the expression of the characters, 

 rather than as differences of transmis- 

 sion. The mathematical formulae of 

 Mcndelism apply as readily to the 

 expression of characters as to trans- 

 mission and the polygenic conception 

 of heredity enables us to avoid the 

 misleading inferences that have been 

 drawn from the monogenic theories. 

 It no longer apjjcars necessary to believe 

 that the degenerate mutations of do- 

 mesticated plants and animals represent 

 new species, or even new characters. 

 Uniformity in the expression of char- 

 acters no longer appears to be the 

 normal condition of heredity or of evo- 

 lutionary progress. On the basis of a 

 polygenic theory the typical condition 

 of heredity, as of evolutionary progress, 

 is found in normally diverse, freely 

 interbreeding wild species, instead of in 

 narrow-bred or self-fertilized domesti- 

 cated varieties. The production of 

 groups of uniform individuals by selec- 

 tive breeding or by special methods of 

 propagation does not eliminate charac- 

 ters from transmission, but only reg- 

 ulates the cx])rcssion of characters. 

 It remains ])ossible for the characters of 

 remote ancestors to reappear in rever- 

 sive mutations. 



That characters should be trans- 

 mitted in a latent condition may seem 

 too wonderful for belief, but the fact 

 remains that all characters are trans- 

 mitted in a latent condition. The 

 geiTn-cells, eggs, embryos, lar\'ae and 

 juvenile states do not have the char- 

 acters of advilts, except in the .sense of 

 transmission, nor do adults have the 

 characters of the preliminary stages, 

 exce]3t in the same sense of trans- 

 mission to their offspring. It need not 



*Cook, O. F., "Dimorphic Leaves of Cotton and Allied Plants in Relation to Heredity," Bulletin 

 221, Bureau of Plant Industry, U. S. Department of Agriculture, 1911. 



'Cook, C). F., "Transmission Inheritance Distinct from Expression Inluritante," Science, n.s. 

 vol. XXV, June 7, 1907. p. 911. 



