BIOLOGY 



possible a minute analysis of the heritable con- 

 stitution, and shown how this may be resolved 

 into factors capable both of being separately 

 transmitted from one generation to the next 

 and of forming permutations and combinations 

 with one another in accordance with the laws of 

 chance. 



The factors whose existence is demanded by 

 the geneticist to explain the results of his breeding 

 experiments have not as yet been identified as 

 visible entities in the germ-cells, but, as I have 

 already indicated, the American school, under the 

 leadership of Thomas Hunt Morgan, have localised 

 them with a high degree of probability if not of 

 certainty in the chromosomes of the nucleus, 

 and even devised a means of mapping out their 

 distribution therein. It is here that the work of 

 the cytologists and that of the geneticists comes so 

 closely into touch, for our recently acquired know- 

 ledge of the behaviour of the chromosomes in 

 cell-division, and especially in the maturation of 

 the germ-cells, and in the process of fertilisation, 

 supplies exactly the mechanism for the permuta- 

 tion and combination of factors required by the 

 Mendelian investigator. 



Arresting and important as these results un- 

 doubtedly are, however, we must be on our 

 guard against supposing that the factorial hypo- 

 thesis can ever provide a complete solution of the 



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