39o THE POPULAR SCIENCE MONTHLY 



appearance is that of changing the type by environmental means, but 

 nothing more than selection has been at work. 2 



From all this we are led to conclude that natural selection is con- 

 tinually operative on the lower types of life, the unicellular animals 

 and plants, everywhere affecting the proportions or existence of the 

 races and species. These creatures are not adapted to life under all 

 conditions; they are, on the contrary, sensitive to relatively slight 

 changes, many of them probably too slight for us to appreciate. The 

 history of a single culture in the laboratory indicates this. Why, then, 

 are the species so widely distributed, and, on the whole, so constant? 

 "Why are they not infinite in number ? Why are they not exterminated 

 in great numbers, instead of being of tremendous antiquity, as their 

 wide distribution and the paleontological records show? Where the 

 environment is highly specialized, as in the case of groups parasitic on 

 the higher vertebrata, there is considerable evidence that evolution has 

 to a certain extent kept pace with that of the hosts ; yet always tending 

 to lag behind, as Kellogg showed even in the case of the bird-lice, 

 which are of far higher organization than the types now under dis- 

 cussion. In the case of such animals as the fresh-water Protozoa, how- 

 ever, the selective processes have always acted piecemeal, rarely if ever 

 sufficiently widely to destroy a species which had once gained a good 

 footing. They have no doubt destroyed many incipient species, but 

 any tolerably successful type, once widespread, may defy the ordinary 

 processes of nature. In a wide country there is nothing which renders 

 every puddle uninhabitable, or every part of each pond and river, and 

 survival in a number of places permits the reappearance of the crea- 

 ture in millions when good conditions for reproduction occur. All that 

 is necessary for permanence is an inherent stability of type, which will 

 prevent automatic modification independent of conditions. This sta- 

 bility surely exists in an amazing degree, and may itself be regarded 

 as a product of selection acting through the ages; for automatic insta- 

 bility, manifested too much or too often, would lead to series of changes 

 eventually fatal to existence. A certain looseness of adjustment to 

 surroundings is advantageous, but even slight variations, piled one 

 upon the other, would before long throw the organism out of gear. 



Perhaps we may picture the condition of affairs somewhat as follows : 

 There are, let us say, 500 common "situations" in the fresh waters 

 of the world, differing in the temperature and chemical content of the 

 water, in the presence or absence of particular enemies, in the quantity 

 and quality of available food, and I know not what else. These are 



2 There is some evidence, the precise value of which can not at present be 

 determined, pointing to a selective destruction of certain germinal elements 

 under special conditions. This, if adequately confirmed, may equally explain 

 some of the results obtained with trypanosomes, and the often-quoted work of 

 Tower and MacDougal in inducing heritable variations. 



