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THE AMERICAN N. I TURA LIST [ Vol. LI 



will be too much bored to make the tests that will dis- 

 tinguish a given new member from some one or other of 

 the old ones. But Jennings may reply, supi>ose the selec- 

 tionist claims that his material is already in this finely 

 triturated condition ! If, so, the answer is tliat by suitable 

 selection experiments an analysis may in many situations 

 still be made, and, secondly, the evidence, even from 

 Castle's rats, is far from establishing that lie is dealing 

 with such a sublimated process. On the contrary, there is 

 much in them to indicate that they may be cav)able of 

 being handled by rather simple Mendelian methods, as 

 MacDowell has shown. 



As a matter of fact, when indistinguishable characters 

 are the product of one or another modifier, the identifica- 

 tion of the two genes involved, as independent, is per- 

 fectly easy and certain by means of linkage relations. If 

 a particular material is not sufficiently worked out to 

 make this test possible, is that a sufficient reason w^hy w^e 

 should refuse to accept evidence where it can be obtained ? 

 And if there are indistinguishable characters that are the 

 product of one or of anothei- allrlomorpli, of course it can 

 not be determined wliicli al1('h)in()i iili produces the result; 

 but as, ex hypothesi, each alh'hiinoi'] >h ])roduces the same 

 indistinguishable result, a (Hsciission of such a question 

 would be as j>rofitable as the ancient one of the number of 

 angels that can stand on the point of a needle. 



In conclusion then it may be said that by stable or 

 constant genes we do not necessarily mean that the gene is 

 absolute in the sense that a molecule is absolute, for we 

 can not know this at present. We might mean by stable 

 lienes that even if there is a variability of the gene the 

 variation takes phice about a mode; and if in a given 



siH)ti,li„u- ,-xtrci,,.. 1,1 the vaiialioii ..f the uviic," >lill the 

 experimental cvidcn.-c ^how> that in the iiianv ccll-i>vnc,-a- 

 tions through which that in.livi.hiars uvi'ni ccll^ pa,^> to 

 produce the spcnii- of the eiius, the ^vnic Nariation. if 

 there is any, is still abont the orif/iiKd mode and that no 



