No. 530] PURE LINES IN STUDY OF GENETICS 



SI 



in the face. We have, side by side in the laboratory, a lot 

 of diverse sets of our organisms, each set derived origin- 

 ally from one individual, and each differing characteris- 

 tically but minutely from the others — the differences per- 

 sisting from generation to generation. The behavior and 

 properties of these things are of course a matter for 

 further study. Can selection change them? Can envi- 

 ronmental action permanently modify them? These are 

 matters quite distinct from the existence of the genotypes. 



To get a clear grasp of the matter, I believe that those 

 not working with lower organisms will find it worth while 

 to try to realize the condition which the investigator in 

 this field has before him. A comparison may help. In 

 lower organisms the genotype is actually isolated, each in 

 a multitude of examples, which live along without admix- 

 ture, visibly different from all others, for many genera- 

 tions, before again plunging into the melting pot of cross- 

 breeding. In higher organisms we should have the same 

 thing if every rabbit, every dog, every human being, 

 multiplied by repeated division into two like itself, till 

 there were whole counties inhabited by persons that were 

 replicas of our absent president ; cities made up of copies 

 of our secretary, and states composed of duplications of 

 the janitor I saw outside. Every human being, as it now 

 stands, represents a different genotype (save perhaps in 

 the case of identical twins), and these genotypes become 

 inextricably interwoven at every generation. It is there- 

 fore easy to see how the genotype idea might appeal to 

 workers among higher organisms as a mere hypothesis. 



What then are these visible, tangible, isolated geno- 

 types (or races, or strains) of lower organisms, and how 

 are they distinguished? Taking Paramecium as a type: 



1. Some of them differ in size — the size of each remain- 

 ing closely constant, under given conditions, for hundreds 

 of generations; for years. This was the first difference 

 observed, and I tried to demonstrate it by giving meas- 

 urements of successive generations of the different races. 

 But to the worker in the laboratory these differences are 

 evident without refined measurements; the student is at 



