May 17, 1907] 



SCIENCE 



793 



This complete demonstration that latent 

 characters, at least in many cases, are not 

 inactive units that may be rendered active by 

 some unknown influence, but are, instead, 

 units that produce a visible character only 

 when acting in conjunction with one or more 

 other units, jiistifies me in calling attention 

 again to the significance of such characters. 



In order to see the bearing of these results 

 upon the process of evolution it is necessary 

 to realize that what we call a unit character 

 is not necessarily produced by the activity of 

 a single allelomorph, and I consider it prob- 

 able that few visible characters are so pro- 

 duced. It makes no difference how many 

 internal units are involved in the production of 

 any so-called unit-character, so long as there 

 is a difference of only one unit involved in . 

 the cross. Thus, allelomorphs ABCDEFGE 

 may determine a single characteristic and 

 ABCDEFOh an alternative characteristic. If 

 plants having characters so determined are 

 crossed together, they will behave as if these 

 were unit characters, though according to our 

 assumption one is determined by the presence 

 of eight dominant units, the other by seven. 



The best actual examples we now have of 

 the compound nature of certain apparently 

 simple external characters are seen in the 

 splendid results of Professor Bateson's studies 

 on stocks and sweet-peas. In stocks, for in- 

 stance, eanescence is found to depend upon the 

 simultaneous presence of three dominant al- 

 lelomorphs wholly uncorrelated and each act- 

 ing in the normal Mendelian manner. In one 

 strain of sweet-peas two such dominant units 

 are necessary to produce any color whatever 

 and another unit determines whether that 

 color shall be blue or red. This condition 

 produces the remarkable result that the first 

 generation hybrid between two white-flowered 

 parents have blue or red flowers. 



Similar conditions were presented in two of 

 the papers given yesterday (December 28, 

 1906) on the joint program of Sections F and 

 G of the American Association for the Ad- 

 vacement of Science, viz., the appearance of a 

 ' latent ' agouti factor in certain guinea-pigs, 

 and an invisible red factor underlying black 



in certain fowls as reported by Dr. Castle^ and 

 Dr. Davenport.* The characters of both these 

 apparently anomalous hybrid products were 

 recognized as atavistic or reversionary. The 

 same is true of the purple-flowered hoary stocks 

 produced from glabrous white and glabrous 

 cream-colored strains. The same was true of 

 flower and seed'-coat color of beans and peas 

 as found by Tschermak and Lock, and is no 

 doubt the correct explanation of the purple 

 mottling in my hybrid beans. Indeed, so 

 many instances are now on record in which a 

 cross results in reversion, that generalizations 

 can be made with some degree of security. 



These reversions indicate that the original 

 character was compound, being determined by 

 the simultaneous action of two or more, pos- 

 sibly many, dominant units, and that the later 

 specific or varietal derivatives were produced 

 by the disappearance of one or more of these 

 original units as a dominant characteristic. 

 Thus in the example assumed above in which 

 the original character was determined by the 

 dominant units ABCDEFGE, the later de- 

 rivatives may be ABCDEFOh, ABCDEFgH, 

 ABCDEfgE, etc., through all the possible per- 

 mutations. May we not perhaps get in this 

 way a comprehensive view of at least the later 

 stages of evolution as a process of analysis 

 due to the disappearance of one unit after 

 another ? 



All the visible variations _ of the present 

 plant and animal world were once involved in 

 some generalized form or forms, and the 

 process of differentiation pictures itself to us 

 as a true process of evolution brought about 

 by the change of individual character-de- 

 termining units from a dominant to a reces- 

 sive state. This conception results in an 

 interesting paradox, namely, the production 

 of a new character by the loss of an old unit. 



When I first became interested in the Men- 

 delian discipline one of the most difficult 

 things for me to understand was the fact that, 

 somehow, every dominant character in a plant 

 or animal finds its recessive counterpart in 



' Castle, W. E., ' On a Case of Reversion In- 

 duced by Cross-breeding and its Fixation.' 

 'Davenport, C. B., 'Reversion.' 



