IOWA ACADEMY OF SCIENCES. 83 



changes in position in relation to it, so that as it develops 

 it may travel into quite different conditions from those 

 prevailing at first. Hence we see that even a slightly dif- 

 ferent response to external forces in the first stages of 

 development, due either to a different protoplasmic struc- 

 ture or to differences in these forces themselves, may pro- 

 duce ultimately important results. To put the case in a 

 strong form I may quote from Ryder: "The initial con-- 

 figuration or mechanical arrangement and successive 

 rearrangements of the molecules of a germ, the addition 

 of new ones by means of growth, plus their chemical and 

 formal transformation as an architecturally self-adjusted 

 aggregate, by means of metabolism, is all that is required 

 in an hypothesis of inheritance/' 



While the theories of pangenesis and of the continuity 

 of the germ plasm are in themselves of great scientific 

 interest, they would probably not have attracted so much 

 attention from people who are not especially devoted to 

 biological problems were it not for their relation to certain 

 theories of evolution. Characters appearing in an indi- 

 vidual duiing development may be regarded as arising from 

 the operation of either one or two sets of forces, those from 

 heredity (such characters being called coiujenital) and 

 those from the environment (such characters being spoken 

 of as acquired). 



Darwin assumes that characters acquired in one genera- 

 tion become, at least to a certain extent, congenital in the 

 next. By the hypothesis of pangenesis this is easily 

 accounted for, since the gemmules are given off from the 

 organs that have been modified during the whole life of 

 the individual, and will hence modify the hereditary sub- 

 stance in the germ after the characters have been acquired. 

 The theory of the continuity of the germ plasm, implying 

 as it does the impossibility of representative changes in the 

 germ plasm, due to modifications of the somatoplasm, does 

 not provide for the transmission of acquired characters. 



Weismann was led, in consequence of this and other 

 considerations, to deny absolutely the possibility of acquired 

 characters being congenital, or in other words, he denied 



