Xo. 5S4] 



If EHKD1TY AX I) ITS ME AX IX a 



487 



In proposing this thesis, the chromosomes have been 

 considered as pairs of freight boats loaded with character 

 determiners, shifted bodily to the daughter cells by in- 

 ternal forces of which we are ignorant. Yet this is not 

 the whole truth. The determiners in particular chromo- 

 somes seem to be tied together more or less tightly, but 

 they are not always transferred as one package. They 

 are coupled in their transmission to the next generation, 

 but this coupling is not perfect. Breaks in the coupling 

 occur and there is order and regularity in these breaks. 

 Our knowledge on these matters rests upon the researches 

 of Morgan on Drosophila, Bateson on the sweet pea, and 

 Tanaka on the silkworm, so it is not certain whether these 

 are common grounds for this regularity or whether each 

 species has regular laws which control the breaks in cor- 

 relation. But in either case, these breaks do not inter- 

 fere with our proposition. They only complicate matters. 

 In most of the cases in Drosophila, where they are best 

 known, linkage is comparatively tight, i. e., breaks are 

 somewhat rare; but they may become so frequent as to 

 simulate inheritance from separate chromosomes. In 

 those cases our theory is of no value, but if Drosophila 

 is any criterion by which to judge, such conditions are 



Abnormal Chromosome Behavior and Heredity 

 The third query concerning the relations of peculiar or 

 unusual chromosome behavior to the transmission of 

 characters may be passed over with a word. In certain 

 insects, for example, bees, wasps, aphids, phylloxerans, 

 etc., odd sex ratios and attendant complexities have long 

 been known. These have been cleared up more or less 

 completely by cytological studies. They depended upon 

 chromosome behaviors that are not usual in animals or 

 plants. Similar peculiar chromosome mechanisms may 

 be present in many other species. Attention is merely 

 called to the fact that if experiments on any plant species 

 appear to show that its characters do not obey the laws 

 that have been demonstrated for so many types, their 



