No. 606] 



BIOLOGICAL ENIGMAS 



346 



However, the facts of "crossing- over" observed in re- 

 cent studies on the relation between Mendelian charac- 

 ters and chromosome constitution'^^ sliow that the latter 

 is not inviolate, even in purely g-erniinal segmentation. 

 Since the power possessed by cells to assume germinal 

 character, even to the limited degree of being able to 

 regenerate a single organ or tissue, seems to vary in in- 

 verse proxiortion to the degree of specialization of the 

 cells, it is reasonable to suppose that Weismann's piin- 

 cii3le of differentiating division actually does operate in 

 the higher stages of development. However, at no stage 

 is it the only mechanism of differentiation, and it cer- 

 tainly is not the primitive means. 



It is possible that cancer represents a return of tissue 

 cells to a germinal or semi-germinal stage, due to the 

 failure of a ''stimulus of ditferentiation" to remain ef- 

 fective. 



The control by the genetic eiiziimrs of the ])osition, size 

 and form of specific tissues n)n>r til\ t'. \\r>\, a quanti- 

 tative regulation of the procc^- »>!' ditTcrciiiiiition, which 

 can be effected by the estaltli^linirMt .-I" .Iclinitc relations 

 lu'lwccii tlie .•lidiiical .-(Mi^lanN of tiu' .-atalxtir rca.-ri.nis 

 andilu'.-oiHlitb.ii^ iin.lrr lii.-i, t luM-nnrM- nf .IrN .lo, u.irnt 

 iUMn-.aril\ plar,- t li.'iii : nnd, m con.l. .•oiitm! nf ilu- |.laiie< 

 of segmentation of the celN. To at1( iiii't i> ^iMM-lliration 

 of the exact process by which thi- hitter t'actoi' ran be 

 governed by the chemical constitution of" the celi-nncU'iis, 

 lies beyond the scope of the presi iit pa])er, hiu it siionld 

 bo pointed out that in the last anai\ si> eiienrwal coii-litn- 

 tion means nothing Imt a definite -palial ai-i-aiiiit'iin^it of 

 electrical forces, m) that there i- notl itm i .ai-a.h.v'ra; in 

 the determination of "inire form" h\ cMeiiiica' a-^fuN. 



As is evident in tlie .nrntation- nueh- tiom i'.ate- 



son, the dominant jn-obh-m in the modem (h-ni^-iuii uf 

 evolution iv that of lh< n,ujni , >,.r,^. It i^ tlie 

 failure of Xeo-Dafwinians to i-xphiin the api-earatiee of 

 variations, and e-peelaliv of new unit eharaeter-. \\\\\v\\ 

 has led sm-h urlter^ a^ Diir^-n. K,.rMdnn^k> and Wolff 



51 See Morgan, h)c. c\t., 



