300 SCIENCE PROGRESS 



mass-action, he must think in terms of equihbrium ; but he does not therefore 

 deny the existence of molecules, ions, atoms, or other units. When an 

 organism is in process of development, it presents the biologist with a problem 

 of equilibrium — hormonic and other interrelation of parts, " Kampf der 

 Telle," and so forth. This does not — or should not — cause us to deny the 

 existence of unit-factors which have, as I believe, provided the basis from 

 which the dynamic process of development starts. Professor MacBride 

 writes almost as if he would like to prevent the geneticist from trying to think 

 physiologically ; but I hope that specialisation has not gone quite so far 

 as yet. 



7. Guyer's work. If there are factors in the germ-plasm concerned with 

 lens-production ; if one or all of these are of the same (or similar) composition 

 as the proteins of the lens ; then we should expect an anti-lens serum to attack 

 these factors ; and further, if the damage caused by this attack is permanent, 

 expect that the lenses of future generations will also be affected. This 

 appears to me to be the natural hypothesis ; but I fail to see how it or any 

 other interpretation can be used either for or against Lamarckism. 



8. Abnormalities of division. All Morgan's recent work goes to show that 

 irregularities of division have nothing to do with the origin of Mendelian 

 mutations, and that the alteration is an extremely localised one, occurring 

 in one very small area (a single factor) of a single linkage-group (on Morgan's 

 view, a chromosome.) 



9. Weismann. My remarks about Weismann were simply intended to 

 relieve Professor MacBride from any necessity of slaying the slain. The 

 continuity of the germ-plasm and the idea of determinants are important 

 broad generalisations. But that there is no interaction whatever between 

 soma and germ-plasm, or that regeneration is " due to " packets of reserve- 

 determinants — such assertions are to-day only of historical interest, whether 

 to Professor MacBride or to Professor Morgan. 



With apologies for taking so much of your space, 



I remain, 



Yours faithfully, 



Julian S. Huxley. 

 New College, Oxford, 

 May 8, 192 1. 



II. From Prof. E. W. MacBride, D.Sc, F.R.S. 



Sir, — Before I enter on the discussion of the points raised in his last letter, 

 on which Mr. Huxley differs from me, I should like to set forth briefly the 

 points on which we agree. Thus, we both beUeve that the chromatin of the 

 nucleus is the bearer of the hereditary qualities, and that, therefore, if the 

 hereditary potencies of any animal are altered, this chromatin must somehow 

 have been affected, 



I believe that the chromatin has been altered in two ways, viz. (a) by the 

 slow influence on it of a changed cytoplasm, the cytoplasm itself having 

 become modified by its reaction to changed conditions of life, and (6) by 

 irregularities of division when two sister chromosomes having paired in the 

 prophases of the meiotic or reducing division, again separate from one 

 another. Mr. Huxley appears to think that his mutations arise like Mel- 

 chisedek, causeless and unconditioned, without father or mother. I should 

 like to call his attention to the fact that the master whom he so slavishly 

 follows, i.e. Prof. Morgan, when confronted by this dilemma, is driven to 

 admit that in the last-named resort the cause for mutations must be sought for 

 in alterations in the cytoplasm. 



The principal point in the paper which I contributed to Science Progress 



