72 IRE AMERICAN NAIVBALISI [VouLI 



eliange. This, it will be recalled, was that the survivors 

 must produce offspring which bear characters like those 

 which had led to the survival. Or, to put the matter 

 crudely, the survivors must transmit their characters to 

 their offspring. In pre-Mendelian days this phase of the 

 subject was always neatly and summarily disposed of by 

 stating, as one of the facts on which the theory of natural 

 selection rested, that ''variations are inherited" or "like 

 produces like." Times have changed. We are a great 

 deal less certain about that particular brand of inheri- 

 tance which the theory of natural selection demands than 

 we were before any one had taken the trouble to make 

 experiments on heredity. The essential difificulty lies 

 here. The differences upon which natural selection di- 

 rectly operates are somatic differences, by hypothesis and 

 in fact. Every worker in genetics has learned since the 

 truly epoch-making researches of Johannsen^^ to be ex- 

 tremely cautious in assuming a priori that any particular 

 somatic difference is so inherited. 



The writer has lately been experimenting with a char- 

 acter which very well illustrates this point. A not infre- 

 quent variation of the single comb in poultry is the ap- 

 pearance, on one or both sides of the comb, of a small 

 excrescence, known technically as a side-sprig. Indi- 

 viduals exhibiting this variation have been selected for 

 breeding purposes. But, so far as the experiments have 

 yet gone, it does not appear that the offspring of such 

 animals are any more likely to exhibit the variation than 

 are the offspring of any random sample of single-combed 

 fowls. Now suppose, for a moment, that in a state of 

 nature the possession of a side-sprig on the comb gave a 

 bird a distinctly better chance for survival than did a 

 plain single comb. Those lacking the variation would 

 then by h\T>othesis tend to be eliminated, but there is not 

 the smallest indication that there would result any pro- 

 gressive evolution towards a side-sprigged race. 



Now one might go on and review a great accumulation 



20 Johannsen, W., "Ueber Erblichkeit in Populationen und in reinen 

 Linien." Jena, 1903. 



