80 THE AMEBIC AN NATUBALIST [Vol. XL V 



from having both sides presented I believe that we have 

 before us. 



What I wish to attempt is to give some concrete illus- 

 trations of the answer to the question discussed by Dr. 

 Webber — What are genotypes ? I note that some of the 

 titles on our program speak of the genotype hypothesis, 

 the pure line theory. What I wish to emphasize is that 

 these things, whatever we call them, are concrete realities 

 —realities as solid as the diverse existence of dogs, cats 

 and horses. I find in many biologists not working in 

 genetics an incorrigible bent for seeking under such a 

 term as genotype something deeply hypothetical or meta- 

 physical, and for characterizing it therefore boldly as 

 purely imaginative. This is merely because such 

 workers have not the things themselves before them. 

 The genotype is merely a race or strain differing heredi- 

 tarily in some manner from other races. Neither the 

 idea nor the fact is a new one, and we should perhaps do 

 better to discuss merely the importance of distinguishing 

 in our work the diverse existing strains — rather than to 

 introduce an unfamiliar term for a familiar thing. But 

 investigation has shown the existence of these strains to 

 play a part of such hitherto unsuspected importance that 

 it has seemed worth while to introduce a more precise 

 term, which shall emphasize their importance for work in 

 genetics. In work with a certain lower organism— 

 Paramecium— 1 have found the existence of these diverse 

 strains or genotypes to be the guiding fact, not only for 

 work in genetics, but for all exact work in comparative 

 physiology. I wish to show how this is true. 



We must then distinguish clearly these concrete 

 realities called genotypes from any theories that have 

 been built up in connection with them ; from any generali- 

 zations based on their study up to this time. The exist- 

 ence and importance of genotypes are not bound up with 

 any particular theory regarding selection or any other 

 single point. In lower organisms, at least, genotypes or 

 pure lines are merely the name for certain actual exist- 

 ences that you have before you ; for facts that strike you 



