Are these Protozoa, then, indifferent to their surroundings? By no means. The experiments of Jennings show that uniformity of environment 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 inspection. 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 Paramæcia in the world. We have, however, the results of nature's large-scale experiment with Paramæcium. The genus, notwithstanding 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 reversed. 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 Paramæcia). Apply some treatment 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
