No. 606] 



METHOD OF GENETICIST 



365 



evolution is once more made wholly speculative. Inas- 

 much as the very instabiHty of protoplasm, which 

 accounts for the manifold metabolic processes that char- 

 acterize living things, makes not only possible but highly 

 probable alterations of the chromosomes which, in our 

 present state of knowledge, must be regarded as of 

 internal origin, the discovery of cases like those of the 

 fruit fly and the evoninii- ])riini()so. in which environ- 

 mental agency is ai)])ar('iitly iiiadtiiissible, should leave 

 no doubt that evoluti(»ii ran occur without reference to 

 si>ecific elements of the outer world. 



What the actual method of producing changes in the 

 chromosomes may be can only be conjectured. Morgan 

 and his students have abundantly demonstrated that the 

 continuous identity of chromosomes is, in at least one 

 animal, an invention based on appearances ; that the 

 chromosomes of one individual are often not identical 

 with those of its parents. The crossing over which they 

 postulate is an even exchange of corresponding parts of 

 two chromosomes. How this exchange is brought about, 

 whether through the twisting of the chromosomes as the 

 students of Drosopliila have assumed, or because of the 

 variability of the forces thai hohl the cliroiiiuMtiiies 

 together, as ( loMx-linii.lt (1!>17) suggot^. is iniinaterlal. 

 If, occasionally, this exchange between the chromosomes 

 is not e(|ual. an occurrence that is not inconceivable, a 

 chrouiosonie might be produced unlike any that ever 

 existed, if the germ cell containing such a chromosome 

 were capable of producing a viable individual, to predict 

 the probable nature of such an organism would be idle 

 speculation. 



Failure of the chromosomes to divide, and the passage 

 of one or more of them bodily to one end of the spindle, 

 would produce daughter cells with an unequal comple- 

 ment of hereditary material. Hyde (1916) has recently 

 reported a case in Di'oso]>hila which is ]>ro1)ably of tjiis 

 nature; the two X chroiiio-onies appear to have remained 

 undivided, goinu' to ()i>|M)>it<' daughter cells, resulting in 



