CORRESPONDENCE 



To THE Editor of " Science Progress " 



THE INHERITANCE OF ACQUIRED CHARACTERS 



I. From Julian S. Huxley, M.A. 



Sir, — May I be allowed to comment, with the utmost possible brevity, 

 upon Professor MacBride's reply to my letter in your last issue ? 



1. Prof. MacBride, in the passage to which I took exception, in reality 

 made two statements : first, that the mutations employed by Mendelians 

 are pathological; and, secondly, that they are not of evolutionary import- 

 ance. In his reply he has dealt only with the second ; the more sweeping 

 and unwarranted first assertion he has not attempted to defend. 



2. Eye-colour . Various species of animals and birds, as well as different 

 races of man, are distinguished by differences of eye-colour, which appear to 

 be or are most easily interpreted as, Mendelian. For the question of degenera- 

 tion Professor MacBride should consult the discussion by Miiller. 



3. Segregation of quantitative factors. Whenever a cross gives an F.i, 

 which is intermediate and has a small range of variability, followed by an 

 F.2, with a large range of variability, segregation is the only known hypothesis 

 which wiU cover the facts. See discussion in Morgan's Mechanism of Men- 

 delian Heredity, or Baur's Einfiihrimg in die experimentellan Vererbungslehre. 



4. Colojirs of Mammals. I accept Professor MacBride's challenge. If he 

 will consult R. E. Lloyd's The Growth of Groups in the Animal Kingdom, 

 he will find evidence of colour-variations in wild rodents which are [a) dis- 

 continuous and presumably segregable, [b) resemble the patterns of other 

 wild species. Again, the difference between white-bellied and grey-bellied 

 types (a common distinction in nature) is in rodents due to two members of a 

 multiple allelomorph series ; and a specific difference in coat-colour between 

 two species of Cavia has been shown to be Mendelian, and the factor for it has 

 actually been transferred from one species to the other. 



When yellow ground-colour (as in many desert forms) and sharply 

 defined patterns (as in Skunks) occur, the onus of proof rests on those who 

 deny that these appearances are due to factors similar to those wliich produce 

 similar effects in domesticated rodents. 



5. Modifying factors. In certain cases these have been definitely shown 

 to exist (Morgan, Carnegie Publication, No. 278, 1919, pt. iv ; Attenburg 

 and Miiller, Genetics, Jan. 1920.) 



Let me ask Professor MacBride to look at Crampton's monumental work 

 upon the snails of the genus Partula in Tahiti {Carnegie Publication No. 228, 

 1 916), and say whether his facts can be explained except by the assumption 

 of variations which are (i) inherited ; (2) accidental, i.e., in no observable 

 relation with the observed differences of environment ; (3) discontinuous ; 

 and (4) of small amount — in other words, small mutations, and often modify- 

 ing factors. 



6. Organic equilibrium. When a chemist is dealing with a problem of 



299 



