Mat 30, 1913] 



SCIENCE 



825 



eated regarding the chromosomes stand, 

 however, upon much firmer ground. That 

 the chromosomes are in fact causal agents 

 in determination can now be doubted, so I 

 think, only by those who refuse to reckon 

 squarely with the whole body of evidence. 

 That the distribution of the chromosomes, 

 or of smaller elements that they contain, 

 gives us at least a partial explanation of 

 the behavior of unit-characters has become 

 in a high degree probable. The stubborn- 

 ness with which each step in the establish- 

 ment of these conclusions has been con- 

 tested has been largely due, I think, to a 

 misapprehension for which the advocates 

 of the chromosome-theorj' are themselves 

 in part responsible. The chromosomes 

 have often been spoken of as if they were 

 central, controlling factors in heredity, or 

 as if they were actual bearers of the unit- 

 characters — the latter form of expression 

 has in fact been employed, for the sake of 

 brevity, in the foregoing discussion. But 

 it seems to me that such expressions are, to 

 say the least, misleading ; they are certainly 

 unnecessary. It is perfectly obvious that 

 chromosomes do not bear hereditary char- 

 acters as such; they bear only somethings 

 that are necessary to the production of 

 characters. I again repeat that these 

 "somethings" may be at bottom of chem- 

 ical nature. We find it convenient, in 

 order to avoid circumlocution, to speak of 

 these things or substances as "determin- 

 ers"; and there is no objection to doing 

 this so long as we do not forget that many 

 other things are concerned in the produc- 

 tion of every character. Experiment has 

 made it certain that the cell protoplasm is 

 thus concerned. It is possible that the 

 chondriosomes or plastosomes may here 

 play an important part. In any case, the 

 conclusion is not to be escaped, I think, 

 that the whole cell-system is directly or in- 

 directly involved in the production of 



every hereditary trait. To treat the 

 chromosomes as if they were central gov- 

 erning or controlling factors in the cell is 

 a procedure of more than doubtful expedi- 

 ency. For the present, at least, all the re- 

 quirements of investigation are sufficiently 

 met if we think of the chromosomes, or that 

 which they carry, only as differential fac- 

 tors in heredity, not as its primary or ex- 

 clusive "determiners." Whether they 

 possess a significance more fundamental 

 than this is a question that may well await 

 the results of further inquiry. 



I can refer here to only one or two of 

 the many disputed questions of detail re- 

 garding the chromosomes. One of the most 

 important is whether the chromosomes re- 

 tain their individuality intact in the nu- 

 clei of the "resting period" or interkinesis 

 that intervenes between successive ceU-di- 

 visions. Some of the most careful recent 

 cytological studies in this direction seem to 

 show that such is not the case. Neverthe- 

 less these same studies, together with recent 

 experimental evidence, give very strong 

 ground for the conclusion that a definite 

 relation of genetic continuity exists be- 

 tween the individual chromosomes of suc- 

 cessive generations of cells. On the one 

 hand, the cytological studies of Boveri, 

 Bonnevie, Vejdovsky and others, almost 

 conclusively prove in certain cases that 

 each chromosome is formed directly from 

 the substance of a corresponding chromo- 

 some in the preceding generation. On the 

 other hand, cytological and experimental 

 evidence combine to show that alterations 

 of the chromosome-groups, involving the 

 addition to or subtraction from the group 

 of one or more particular chromosomes, are 

 perpetuated generation after generation of 

 cells, even throughout the life of the in- 

 dividual. Nature performs such an ex- 

 periment every day in the production of 

 sex; for the particular chromosome-com- 



