WEISMANNISM AND OTHER THEORIES 409 



present attempt to explain heredity in terms of the cell," for upon them 

 is founded the Factorial Hypothesis, now supported by a large mass of 

 experimental evidence. 



In our conception of the nature of the heredity units or factors we 

 have departed widely from Weismann. For him each of the ids arranged 

 in a series in the chromosome represented the sum of the characters of a 

 complete organism; the smaller parts were represented by the smaller 

 units (determinants) composing the id, and these units in turn were made 

 up of biophores, which were ultimate and independent living particles. 

 According to our modern hypothesis each of the serially arranged factors 

 or genes exerts an influence on the development of one or more characters, 

 but does not stand for a complete organism as did the id, or for a part 

 of it as did the determinant. Moreover, it is generally regarded as a mass 

 of some complex chemical substance whose activities are due to its defi- 

 nite though imperfectly known physico-chemical properties, rather than 

 to forces exerted by hypothetical vital units. 



In justice to Weismann it should be pointed out that the frequently 

 made criticism that his theory was a vitalistic one is warranted only to a 

 limited extent. Although his ultimate hereditary units, the biophores, 

 were regarded as actually living particles, Weismann stated that "they 

 are not composed in their turn of living particles, but only of molecules, 

 whose chemical constitution, combination, and arrangement are such as 

 to give rise to the phenomena of life." He was careful to point out that 

 in spite of the fact that it cannot, be proved that no peculiar vitalistic 

 principle exists, we should hold fast to a purely physico-chemical basis 

 of life "until it is shown that it is not sufficient to explain the facts, thus 

 following the fundamental rule that natural science must not assume 

 unknown forces until the known ones are proved insufficient . . . We 

 can quite well believe that an organic substance of exactly proportioned 

 composition exists, in which the fundamental phenomena of all life 

 combustion with simultaneous renewal must take place under certain 

 conditions by virtue of its composition" (1902, lecture 36). 



The manner in which hereditary factors are segregated at gameto- 

 geriesis has been found to be different from that conjectured by Weismann. 

 As indicated in the chapter on reduction, he supposed it to occur through 

 a transverse division of the chromosome, whereas it is now known that 

 it is accomplished by the disjunction of pa.irs of entire chromosomes, 

 the separating members of each pair being qualitatively different. The 

 "reduction" predicted by Weismann was found to occur, but not in the 

 manner he supposed. As shown above, his idea of a further qualitative 

 segregation of units of a lower order in the somatic divisions has not 

 been substantiated. Notwithstanding the abandonment of his theory 

 of development and the changes made in his theory of heredity, Weis- 

 mann's influence on both cytology and genetics was enormous, largely 



