appearance is that of changing the type by environmental means, but nothing more than selection has been at work.[1]
From all this we are led to conclude that natural selection is continually 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 discussion. In the case of such animals as the fresh-water Protozoa, however, 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 creature 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 stability surely exists in an amazing degree, and may itself be regarded as a product of selection acting through the ages; for automatic instability, 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
- ↑ 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.
