94 Life and Death, Heredity and Evolution 



media. Later the parts were brought together again, under 

 uniform conditions, to see whether they would still unite, 

 or whether they had become sufficiently diverse to remain 

 distinct. 



It was found that the parts might be cultivated for a long 

 time under diverse conditions, without becoming so changed 

 as to refuse to unite. The first investigator who studied 

 the matter did not succeed in getting a single race to 

 divide into two that were diverse. 



Wolf continued the experiments, keeping the parts sep- 

 arated a longer time, and using many diverse cultural con- 

 ditions ; in particular he added various sorts of chemicals to 

 the different cultures. In this way after long periods dif- 

 ferences were produced within a single race. An example 

 will make clear the important facts. In one experiment a 

 single original race was divided into eight parts, which were 

 cultivated on diverse media. At intervals it is necessary 

 to transfer each stock to a new lot of its medium, in order 

 to keep the organisms healthy, and it is convenient to use 

 the number of transfers made as a measure of the relative 

 time required for changes to occur. After a few transfers, 

 each of the eight divisions was tried with all the others, and 

 it was found that all would unite readily. This still oc- 

 curred after fifteen transfers. After twenty-five transfers, 

 it was found that a few of the parts refused to unite with 

 some of the others. After thirty transfers there were re- 

 fusals in more than half of the combinations, and after 

 thirty-five transfers each division refused to unite with any 

 of the others. Each of the eight parts of the original race 

 had now become diverse from each of the others ; eight dif- 

 ferent stocks had been produced from one. Before the 

 change was complete there were many transitional condi- 

 tions, in which there was a reluctance to unite, without a 



