Holmes: Unit Characters 



475 



main unaffected? If tip of ears and 

 hind feet show parallel variations in 

 color does it not suggest that we arc 

 here dealing with a sort of outcropping 

 of a color variation which is really 

 present, but less conspicuously expressed 

 in other parts of the skin ? That bodily 

 changes of a general nature may 

 manifest themselves to ordinary ob- 

 servation in one or at least a very, few 

 characteristics is clearly shown in the 

 effects of many diseases. Infectious 

 diseases may have their characteristic 

 s\nTiptoms in certain form-changes while 

 leaving the rest of the body apparently 

 unaffected. Hutchinson's teeth^ in 

 children for instance are the index of a 

 general bodily disease which may have 

 no other very obvious sign. 



Introduce some toxin of disease into 

 the body and you produce certain 

 specific characters. Introduce a change 

 affecting all the cells and certain parts 

 only will reveal the fact by noticeable 

 modifications. The appearance of in- 

 dependent variability of parts may thus 

 result from variations that are in reality 

 organismal in their extent. Not only 

 have so-called particular variations not 

 been studied sufficiently to establish the 

 fact that they are really independent, 

 but numerous cases are known in which 

 variations which to casual observations 

 would seem to affect but a single part, 

 are nevertheless correlated with minor 

 changes of wide extent. We contend 

 therefore that the alleged independent 

 variability of parts upon which Darwin, 

 De Vries, Weismann and others have 

 based so much of their argument for the 

 existence of discrete germinal units 

 rests upon an insecure foundation. 



INDEPENDENT TRANSMISSION 



The question of the independent 

 transmission of characters may be dealt 

 with more briefly. Owing to the inde- 

 pendent way in which so-called char- 

 acters such as tallness and dwarf ness, 

 flower color, characters of seed coat and 

 various other parts of peas may be 

 separated and combined almost at will 

 according to the fancy of the breeder it 



has become customary to look upon 

 these characters as discrete entities 

 borne by discrete elements in the germ 

 cells, and to consider the organism as a 

 mosaic of independently heritable parts. 

 From this viewpoint organisms may be 

 likened to brick buildings in which the 

 bricks may be taken out and replaced 

 by others without materially affecting, 

 except secondarily, the bricks which 

 make up the rest of the structure. But 

 although the facts of Mendelian in- 

 heritance are usually interpreted ac- 

 cording to the mosaic conception, they 

 do not I believe necessitate the adoption 

 of this standpoint. When the Anlage'^ 

 of a green pea is separated from that of a 

 yellow one we are not compelled to 

 assume that something in the germ cell 

 that stands for just greenness is separ- 

 ated from something that is the repre- 

 sentative of mere yellowness. We need 

 assume only that what are separated 

 are the Anlagen of organisms as wholes 

 possessing the characteristics in ques- 

 tion. In other words green and yellow 

 represent organismal variations; ex- 

 pressed in Weismannian terms, green 

 and yellow depend not on determinants, 

 but upon ids, the hereditary bases of 

 whole organisms. 



The logical consequence of this stand- 

 point we have presented is that all 

 Mendelian characters are really general 

 and constitutional, however they may 

 appear to be limited to a particular 

 feature of the organism. Many Men- 

 delian characters are quite patently 

 constitutional while others are ap- 

 parently very limited in their extent 

 like pea and rose comb in poultry. 

 Attention has been so taken up with 

 characters per se that I doubt if much 

 careful scrutiny has been given to the 

 possible correlations of characters in 

 other parts of the body. Has anyone 

 for instance very carefully looked for 

 any more general attributes which may 

 be associated with pea or rose comb, or 

 with the smooth and wrinkled coats of 

 peas? Correlation^'may be difficult to 

 detect, not only for the reasons pre- 

 viouslv mentioned, but because the 



' Hutchinson's teeth are a form of incisor teeth indicativa of hereditary syphiUs. 

 ■* Anlage is a German term much used by genetists to denote the hypothetical something in 

 the germ-cells which determines the nature of a given part of the adult organism. — The Editor. 



