INHERITANCE IN MULTICELLULAR FORMS 151 



growth on the part of the idioplasm of these outer cells, that 

 idioplasm tends to be altered in its constitution as compared 

 with the idioplasm of the centrally situated cells. So that the 

 results of division of the peripheral cells, if retained in the same 

 environment, will tend to produce the characters impressed 

 upon the parent cells, and, if this subjection to the special set 

 of conditions is continued and impressed upon this order of cells 

 for a sufficiently long period or with sufficient intensity, even 

 when pressed or passing into other environment, the cell genera- 

 tions will retain these modified conditions, and, for example, 

 cells of mesoblastic origin will tend to maintain characters 

 different from those derived from epiblast, and this whatever 

 the ultimate position of the cells in question. 



In fact, as a first law, we may lay down with Driesch that 

 the structure of the cells in a multicellular organism is a function 

 of their position, and this because the position of the cell deter- 

 mines the modification undergone by its idioplasm. As a second 

 law we may lay down that the greater the change impressed 

 upon the idioplasm of these cells, and the longer that idioplasm 

 is subjected to the conditions inducing this change, the more 

 permanently will the daughter cells exhibit the peculiar altera- 

 tion in the idioplasm, with consequent modified structure wher- 

 ever they find themselves in the economy. We have, in short, 

 to recognize that two orders of forces determine the structure 

 of every cell in the body : (1) the previous influences acting 

 upon its idioplasm and causing it to be of a particular chemical 

 constitution ; and (2) the position in which the cell finds itself, 

 and the forces acting momentarily and immediately upon its 

 idioplasm. Or, briefly, these two series of forces are inheritance 

 and environment, and inheritance and environment determine 

 the constitution of the idioplasm and the structure of the cells. 1 



1 To the worker in bacteriology the hesitancy on the part of biologists to 

 accept environment as a most important factor in originating variation is 

 almost incomprehensible. Nothing is more remarkable in the study of the 

 lowest unicellular forms of life than this effect of environment in bringing 

 about changes in character, changes which not only tell upon a limited series 

 of generations (as I have already pointed out) but are permanent in their 

 nature. By no means save altered environment could Hansen (Compte rendu 

 des travaux du laboratoire de Carlsberg, v., 1900, 1 ; Abstr. in Ctbl. f. Bakt. 

 2te Abt. vii., 1901, 199), taking isolated yeast cells or " spores " (100 per cent 

 of which, when cultivated under ordinary conditions, gave rise to spore-bearing 

 forms) and subjecting these to the highest temperature at which growth could 

 still occur, obtain a race or variety of yeast which now, after twelve years, 



