July 17, 1908] 



SCIENCE 



87 



transmitted in a perfect state to some of the 

 offspring and not to others — such as distinct col- 

 ors, nakedness of skin, smoothness of leaves, ab- 

 sence of horns or tail, additional toes, pelorism, 

 dwarfed structure, etc. — have all been known to 

 appear suddenly in individual animals and plants. 

 From this fact, and from the several alight, aggre- 

 gated differences which distinguish domestic races 

 and species from one another, not being liable to 

 this peculiar form of transmission, we may con- 

 clude that it is in some way connected with the 

 sudden appearance of the characters in question. 

 . . . Some few characters, however, are in- 

 capable of fusion, but these are unimportant, as 

 they are often of a semi-monstrous nature and 

 have appeared suddenly.'' 



Writers on Mendelism have charitably as- 

 sumed that only the accidental oversight of 

 Mendel's vpritings kept Darwin from appre- 

 ciating the new "principles of heredity." 

 But in reality Darwin was acquainted with a 

 much larger range of Mendelian facts than 

 Mendel himself. Even the Mendelian propor- 

 tions in the repr^entation of the parental 

 characters were not unknowm to Darwin. 

 Thus he found that reciprocal crosses be- 

 tween syuunetricol and unsymmetrical snap- 

 dragons yielded only the ordinary unsym- 

 metrical types of flowers in the first genera- 

 tion, while in about one quarter of the next 

 generation (37 plants out of 127) the sym- 

 metrical character returned to expression. 



Whether Darwin supposed that such nro- 

 portions would remain regular in particular 

 cases, does not appear, but there is no reason 

 to believe that more knowledge on this point 

 would have altered his conclusions, for he had 

 facts to show that a general diversity of pro- 

 portions attends " this peculiar form of trans- 

 mission." 



The proportions in which the parental char- 

 acters are shown in Mendelian hybrids are 

 not more exact than in the inheritance of 

 sexual characters. Sex-inheritance is cer- 



eaoh successive generation there has been a tend- 

 ency to reproduce the character in question, which 

 at last, under unknown favorable conditions, 

 gains an ascendancy." 



' " The Variation of Animals and Plants xmder 

 Domestication," Chapters XV. and XIX. 



tainly a form of alternative expression, for 

 the secondary characters of one sex are known 

 in many instances to have been transmitted 

 through the opposite sex. Characteristics of 

 one sex can even be brought to expression 

 in the other sex, as a result of castration, 

 parasitism and disease. In sex-inheritance 

 the contrasted characters of the parents secure 

 expression in equal numbers of the offspring. 

 In typical Mendelian inheritance the propor- 

 tions are three to one, but the percentages 

 are variable and are connected by intermediate 

 numbers. 



Mendelism is not a general phenomenon in 

 nature, nor is it confined to distinct groups 

 of animals or plants, or to particular kinds 

 of characters. The Mendelian proportions 

 simply mark one condition or stage of ad- 

 justment of variable physiological functions 

 whose results can be traced from ordinary 

 graded and blended expressions of parental 

 differences, through many degrees of alterna- 

 tive expression, until they reach the highly 

 specialized form of inheritance shown in 

 sexual characters.' 



Transmission is distinct from expression, 

 just as the imprinting of an invisible image 

 on a photographic plate by the light is dis- 

 tinct from the subsequent development of a 

 visible image by solutions of chemicals. 

 With organisms, as with photographs, dif- 

 ferent methods and conditions of develop- 

 ment can bring different results from the 

 same beginnings. The differences arise from 

 the relations that govern expression, instead 

 of from differences in the characters or in 

 the methods of transmission. Reversions, 

 mutations, sexual and Mendelian differences, 

 and even the so-called environmental varia- 

 tions, can all be understood as varied com- 

 binations and degrees of expression of char- 

 acters equally and impartially transmitted.' 



Alternation in the expression of characters 

 is an elective alternation, a choice among the 

 transmitted characters of those that are 



" " Mendelism and Other Methods of Descent," 

 Proc. Washington Acad. Set., 9: 189-240. 



* " Transmission Inheritance Distinct from Ex- 

 pression Inheritance," Science, N. S., 25: 911, 

 1907. 



