No. 636] GKXKTlc si-:(;iu:(:.\Tl(>\ 11 



among the pollen grains. Meanwhile, it must be granted 

 that no indication that gametic linkage results from 

 somatic differentiation has yet been obtained. 



When, however, we examine the view that linkage of 

 factors is a consequence of their association in a chromo- 

 some, we must observe that there is no body of evidence 

 that the number of linkage-systems agrees with that of 

 the chromosomes, a primary postulate of Morgan's 

 theory. Drosopkila is the only example which has been 

 adequately investigated. The cytological appearances 

 are not readily consistent with the other postulate of 

 Morgan's case, that crossing-over is effected by anasto- 

 mosis of chromosomes and exchange of materials be- 

 tween them. In our present ignorance of the physical 

 nature of the factors we are not obliged to assume that 

 an actual transference of material is an essential condi- 

 tion for an exchange of properties; but since Morgan's 

 suggestion is made in that form we are bound to notice 

 how difficult it is to interpret the visible phenomena of 

 cytology in accordance with that hypothesis. Without 

 personal familiarity with etiology no one can have a 

 confident opinion. I observe, however, that in his most 

 recent publication on these subjects, E. B. Wilson" gives 

 a very emphatic "counsel of caution," remarking that 

 writers on genetics have taken too much for granted, 

 and that for the present the genetic development of the 

 chromosome theory has far outrun the c5i;ological. To 

 a la>Tnan the visible appearance of chromosomes is 

 scarcely suggestive of the prodigious; material hetero- 

 geneity demanded, and the general eoui -c of c \ t.tlouical 

 evidence seems to indicate that the rolr of tiir fliromo- 

 somes is passive rather than active. TlKniuh -howing 

 features of regularity, they are eajnihlc of xcrv wide 

 variations without transgressing the limits of \ iahility, 

 which could scarcely be the case were every detail in 

 their organization critical. Tlie api-cai aiice of chromo- 

 somes is not to me Hi-uotivi' of >ti iii,uv^ of beads of ex- 

 treme heterogeneity. l)ut ratlu'i- of straii(l> of s^me more 



