Cook: Reticular Heredity 



343 



internal organs of the germ-cells might 

 represent actual, visible units of hered- 

 ity, have proved very attractive, and 

 have induced many recent writers to 

 abandon the Darwinian conception of 

 gradual evolutionary progress in nat- 

 ural species. Darwin was familiar with 

 abrupt changes of characters, or sports, 

 as he called them, but he did not con- 

 sider them new species or examples of 

 nornial evolutionary progress. The 

 doctrine of De Vries, supported, as it 

 seemed to be, by the Mendelian theory 

 of heredity, has been hailed as a great 

 advance over Darwinism. But in some 

 respects the new anti- Darwinian theories 

 are logically inconsistent among them- 

 selves as well as lacking the support of 

 pertinent facts. The more we emphasize 

 the idea of characters as represented by 

 definite pre-existent entities or gens, the 

 ifiore difficult it becomes to believe that 

 new characters are suddenly implanted. 



That workers in other fields of biology 

 have been unable to accept the recent 

 Mendelian and De Vriesian inter- 

 pretations of the facts of heredity has 

 not seemed a serious matter to those 

 who were fully convinced by the sta- 

 tistical and cytological arguments. 

 From this point of view it seemed a 

 waste of time for "geneticists" to 

 familiarize themselves with the natural 

 species from which Darwin and his 

 successors attempted to learn the nature 

 of evolutionary processes. Indeed some 

 writers have formally dismissed the 

 idea of species as too vague and indefi- 

 nite for the application of "exact 

 methods." Instead of attempting to 

 understand the development of new 

 characters through the gradual evo- 

 lutionary progress of species, attention 

 is now given mostly to the changes 

 of characters that appear in artificial 

 pure bred strains under "experimental 

 conditions." 



But even this class of variations does 

 not serve to support the monogenic 

 conception of heredity. There is still 

 no adequate proof of the most ele- 

 mentary assumption of the monogenic 

 theories regarding hybrids and muta- 



tions, that one gen or character-germ 

 can be taken out and another substi- 

 tuted, and leave the remaining charac- 

 ters the same as before. Mutations and 

 hybrids have not been found to differ 

 from each other by single characters 

 as they are supposed' to do in Mendelian 

 and De Vriesian systems. 



The fact that mutations commonly 

 differ by many characters, instead of 

 by single characters, was noticed several 

 years ago, and was stated in a discussion 

 of the difference between varieties and 

 natural species^ Since that time many 

 additional mutations and h^'brids of 

 cotton and other plants have been 

 examined with this question in mind, 

 but thus far without finding a satis- 

 factory instance of monogenic .muta- 

 tion or mutative reversion. 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 

 should 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. 



ABNORMAL MENDELIAN PHENOMENA 



The nearest approach to monogenic 

 behavior is found in cases of albinism 

 or similar differences that may arise 

 from simple chemical reactions. But 

 certainly these color changes and allied 

 phenomena afford a very narrow margin 

 for the support of the general Mendelian 

 idea of the existence of characters as 

 independent units. Indeed, many Men- 

 delian writers have virtually abandoned 

 this idea of alternative units in adopting 

 the "presence-and-absence" hypothesis 

 to explain the results of Mendelian 

 experiments. But if it be assumed that 

 the Mendelian condition of alternative 

 inheritance is a consequence of the 

 dropping of normal characters out of 

 expression, the Mendelian phenomena 

 lose all constructive evolutionary signif- 

 icance and take their place among ad- 

 mitted abnormalities.' 



-Cook, O. F., Mendelism and other Methods of Descent, Proc. Washington Academy of Sciences, 

 9: 235. 1907. 



^Professor Castle has recently utilized this view of Mendelian characters as affording a basis for 

 simplification of the Mendelian notation. See Castle, W. E. Simplification of Mendelian 

 Formulae. Amer. Nat. 47: 170, 1913. 



