Page:Popular Science Monthly Volume 84.djvu/133

Rh be quite out of the question to do more than determine whether the death rate is selective or random, and in such cases these first steps may be of very high importance indeed. Again, it may be feasible to plunge at once into the second problem by testing the value of character after character in the battle for life. In this case both phases are simultaneously taken up. These points will be made clear by illustrations.

The first problem to be taken up is therefore that of the existence or non-existence of selective mortality. In a considerable range of living forms it is desirable to know whether natural selection is operative, even though it is for the time being out of the question to say how it is operative, i. e., what particular characteristics make for incapacity or for fitness.

A. The Simple Demonstration of the Existence of a Selective Mortality Studies on Plants.—A first illustration of the importance of the simple determination of the selective or non-selective nature of the death rate is to be seen in the cases of the northward extension of cereals or other cultivated plants. At present, very little is definitely known concerning the factors actually involved. It has been frequently assumed that natural selection through the agency of cold or of the shortness of the growing season has been one of these. This view seems to be supported by Waldron's work on alfalfa. He shows that some strains are more resistant to cold than others, and that in the north the less resistant are eliminated. This is all that is necessary to bring about adaptation—which already exists in some strains. Another most interesting piece of work differing widely in material and detail, but depending upon the same kind of reasoning, is that of Montgomery. Our common cereals have been cultivated for hundreds or thousands of years with practically no attention to selection or grading until recent times. He suggests that under the system of planting two or three times as many seeds as can possibly come to maturity, a slow development has taken place through a continuous natural selection with the survival of the strongest.

He has several interesting results for competition, but his most conclusive experiments for selection are those with maize. Planting