518 Varietäten, Descendenz, Hybriden. 



Cook, O. F., Reticular heredity. (Journ. of Heredity. V. p. 

 341—347. 1914.) 



The author characterizes the Mendelian theory of heredity as 

 as raonogenic because it assumes the transmission of only one set 

 of gens or character-units in each germ cell. But this theory does 

 not provide for the transmission of additional gens to represent 

 characters that may not be brought into expression, or that may 

 give rise to unexpected variations in later generations. If a sudden 

 change of characters appears in a member of an otherwise uniform 

 "pure-bred" stock, it is assumed that a new character has been for- 

 med and that such changes in the characters of uniform groups are 

 examples of normal evolutionary progress. The effect of these theo- 

 ries is to return to the pre Darwinian doctrine of special creation, 

 except that the species are not supposed to be made altogether de 

 novo, but by implanting the gens of new characters in members 

 of old species. In all cases where a definite change in one character 

 was noticed equally definite differences in other characters were 

 found. Such facts indicate that mutative changes shculd be thought 

 of as polygenic, that is, as affecting several characters at once, 

 instead of assuming that they are commonly or typically monogenic, 

 in the sense of being limited to Single characters. 



In reaUty alternative inheritance is not limited to characters that 

 show the Mendelian reactions, but extends into other fields of he- 

 redity where the theory of germinal segregation and alternative 

 transmission of unit characters does not apply. Changes of character, 

 occurring during the development of the same individual plant or 

 animal, must represent alternative expression of characters, instead 

 of alternative transmission. It appears reasonable to expect that 

 sudden changes or abrupt differences among the individuals of the 

 same stock may represent variations in the expression of characters, 

 rather than differences of transmission. Changes of expression must 

 be considered in the study of heredity, as well as differences of 

 transmission. 



A polygenic theory is required to account for the fact that 

 latent or suppressed characters are transmitted, as well as those 

 that are brought into expression. A multiple transmission of many 

 alternative gens should be recognized, enough to represent the 

 whole ränge of ancestral diversities The polygenic nature of here- 

 dity is indicated by the fact that mutative changes of expression 

 usually affect many characters at once, instead of Single characters. 

 A further advantage of the polj^'genic conception is that it allows 

 characters to be brought into analogy with ancestral lines that 

 form the network of descent of the species. 



Merely increasing the numbers of gen sdoes not fully provide 

 for the fact that many characters are usually changed together 

 instead of one character at a time. Our ideas of relations of the 

 gens to each other have to be modified in addition to recognizing 

 increased numbers of gens. If we think of characters as representing 

 stages and alternative courses of development, the gens, as prede- 

 termining rudiments of the characters, must be thought of a having 

 intimate mutual relations, instead of being considered as entirely 

 independent. The idea of alternative courses of development is in 

 better accord with the general facts of biology than the idea of 

 alternative gens, as shown by the nature of individual differences 

 among the members of normally diverse species, with free inter- 

 crossing of different lines of descent. Instead of trying to think ot 



