Page:Popular Science Monthly Volume 78.djvu/534

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524
THE POPULAR SCIENCE MONTHLY

conditions. Concerning the fate of any individual we can say nothing. The problem is, therefore, a statistical one; evolution, as it occurs in nature, is not a problem of "individuals" but of "populations."

 

A. The Protective Value of Color

For a century field naturalists have observed the close similarity between the colors of organisms and their environment, and have seen in this resemblance an adaptation for protection. Since the advent of the selection theory protective and aggressive resemblance, warning colors, recognition characters and mimicry have been prominent in biological literature, and are conceptions associated with some of the most honored names in biology. Yet almost all the evidence has been comparative, and attempts to determine empirically whether given color patterns are in the long run of vital significance are discouragingly few, and some biologists are now questioning whether the so-called protective adaptations have any value at all.

One of the simplest and most direct tests of the value of any character in determining the chances of survival of an individual is that of Di Cesnola for the protective value of color in Mantis religiosa[1] In Italy the green individuals of this species are found on green grass, the brown ones upon grass burnt by the sun. If the color has any protective value there should be a higher death rate from enemies when the insects are exposed on vegetation of a color unlike their own.

Altogether 110 insects, 45 green and 65 brown were secured and were exposed on separate plants as follows:

Green insects on green plants 20
Green insects on brown plants 25
Brown insects on brown plants 20
Brown insects on green plants 45

The experiment began August 15 and observations were made daily for seventeen days. Of the forty individuals exposed on vegetation of similar color, every one survived throughout the entire experiment. All the green individuals exposed on brown grass were killed in eleven days; of the forty-five brown individuals exposed on green grass, ten survived to September 1, when a severe gale destroyed the experiment. This is all made clear by Fig. 1.

The biometrician would like to see this experiment carried out on a much larger scale, but when we consider that not one of the forty insects exposed on similarly colored vegetation was killed at the end of seventeen days, while sixty of the seventy exposed on dissimilarly colored vegetation were eliminated[2] by the end of the first eleven days. I

  1. Di Cesnola, A. P., "Preliminary Note on the Protective Colour in Mantis religiosa," Biometrika, Vol. III., pp. 58-59, 1909.
  2. Most of the insects were destroyed by birds; five were known to have been killed by ants.