642 SCIENCE PROGRESS 



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



Dear Sir, — I am deeply indebted to you for your courtesy in allowing me 

 to see, in proof, the letter of Dr. Julian Huxley criticising my article on 

 the inheritance of acquired characters which appeared in your issue of 

 January last. 



Dr. Huxley's criticisms may be summarised as follows : 



(a) He accuses me of belittling the achievements of Mendelian investi- 

 gators, and objects in particular to the statement that " the pathological 

 mutants which Mendelians employ for their experiments are utterly unlike 

 the variations which the study of comparative anatomy induces us to 

 postulate as the material of evolution." 



[b) He criticises the evidence which I have brought forward in favour 

 of the truth of the Lamarckian doctrine of the inheritance of acquired 

 characters. 



Let us deal with the first point to begin with. Dr. Huxley gives a list 

 of characters distinguishing the mutant from the type which he asserts to 

 be normal — i.e. if I understand him aright, to be comparable to the differences 

 which separate aUied species in nature. Not one of these will stand exami- 

 nation, and some are positively ludicrous. Amongst these characters occur 

 " sex," Does Dr. Huxley seriously wish us to believe that the difference 

 between male and female is one analogous to that separating two allied 

 species ? I have not space to deal with all the instances mentioned, but 

 shall try to expose one or two. For instance. Dr. Huxley mentions " every 

 shade of eye-colour in Drosophila." Now the normal eye-pigment necessary 

 for vision in the eyes of this fly is of a dark red colour. In Morgan's cultures, 

 however, " mutants " appeared in which the red pigment had suffered 

 successive degrees of degradation, giving rise to insects with eyes of colour 

 termed cherry, eosin, cream, ecru, and finally pure white. Dr. Allen has 

 encountered very similar pathological mutants in his study of the eye of 

 the Crustacean Gammarus. If, however, we compare these degenerate 

 eyes with the vestigial eyes characteristic of natural species, we find no 

 resemblance whatever. In the Amphibian Proteus the eye in the adult is 

 a mere vestige, but it shows no regression whatever towards albinism. 

 As Dr. Huxley is perhaps aware, Kammerer has shown that, by exposing 

 the eye of the larva of Proteus to suitable illumination, it can be made to 

 grow into a well-developed eye. 



Size is another of the characters which, according to Dr. Huxley, " segre- 

 gate according to Mendelian laws." But Dr. Bateson, in his Croonian lecture 

 on segregation, states, " Merely quantitative differences seldom, if ever, have 

 a perfectly simple inheritance " ; and again, " It may almost be said to be 

 characteristic of purely quantitative distinctions that one or other of the 

 original parental types fails to reappear in its extreme form after a cross." 

 Dr. Bateson goes on to state that, where segregation of quantitative differences 

 appears to be complete, the quantitative difference is found to be a secondary 

 result of some substantive factor. 



Again, Dr. Huxley mentions " the finest gradation of coat colour and 

 marking in rodents." Does Dr. Huxley seriously believe that any natural 

 races of mice are characterised by the colours shown by these mutants ? 

 Are pure black mice (back and belly showing the same colour), chocolate, 

 or yellow mice ever found in nature as natural species ? The question 

 answers itself — the colours of natural races show the general features of 

 the coat-colour found in the wild mouse, and of no other. 



Let me answer two further objections before passing on to the next 

 point. Dr. Huxley states that I " leave out of account all reference to 

 modifying factors." Now, it is possible to " explain " any given difference 

 between two types as due to Mendehan factors, provided we assume a 



