NATURAL SELECTION 389 



Are these Protozoa, then, indifferent to their surroundings? By 

 no means. The experiments of Jennings show that uniformity of en- 

 vironment is not possible even in a watch glass, and that the animals 

 respond readily in many ways to the conditions under which they exist. 

 This fact has tended to obscure the genetic independence of different 

 races, the characters of which overlap, but "pure line" cultures have 

 made it possible to separate them. It has been shown that distinct 

 races may differ only in average characters, a large proportion of the 

 individuals, under ordinary conditions, being indistinguishable by in- 

 spection. Such a pair of races would only show absolute differences 

 if subjected to conditions ensuring for every individual the maximum 

 or the minimum growth and efficiency. Such conditions are practically 

 unattainable, and only " pure line " breeding and statistical study will 

 separate the races. 



Consequently, in the Protozoa, we have three recognized grades : 



1. The species of ordinary taxonomic writings. 



2. The minor types recognizable by inspection when the investigator 

 is very expert. 



3. The races or strains separable only by breeding combined with 

 statistical study. 



Do the third originate frequently without evident cause? Do they 

 then turn into the second, and the second into the first ? Jennings did 

 not find it so, but his experiments necessarily occupied a limited time 

 and were concerned with an infinitesimal fraction of the unthinkable 

 myriads of Paramoscia in the world. We have, however, the results of 

 nature's large-scale experiment with Paramecium. The genus, not- 

 withstanding its universal distribution and the very diverse conditions 

 under which it must exist, is very poor in species. Either the imagined 

 process does not go on, or it fails before reaching the stage of species- 

 formation, as species are understood by the taxonomists. 



In the ease of bacteria, and even trypanosomes, it is commonly alleged 

 that environment will change the type. This is constantly asserted by 

 the highest medical authorities, and in a certain pragmatic sense it is 

 of course true. It is found, however, that if the environmental factor 

 is carried too far, or continued too long, the process can not be re- 

 versed. It seems nearly certain that the observed phenomena are due 

 to nothing more than a selective process operating on a mixture of 

 races, isolating the one least able to endure. Thus, suppose in a given 

 case we have a culture consisting of one million pathogenic bacteria 

 and ten of an allied non-pathogenic race (presumably there will usually 

 be several grades or races, as with the Paramcecia) . Apply some treat- 

 ment favorable to the ten and destructive to the million, and presently 

 the ten are a million and the million reduced to ten or none. The 



