Page:Popular Science Monthly Volume 78.djvu/537

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527
MEASUREMENT OF NATURAL SELECTION

Fig. 2 will be recognized at once, even by the reader whose knowledge of marine biology is limited to the menu-fauna of the city restaurant as the outline of the solid upper portion of the crab's body known as the carapace. In measuring the frontal breadth[1] of Carcinus from

PSM V78 D537 Carapace of the shore crab carcinus maenas.png

Fig. 2. Outline of the upper surface of the Carapace of the Shore Crab, Carcinus mænas.

a particular spot of beach near the Marine Biological Laboratory at Plymouth, Weldon and Thompson noticed a peculiar change from year to year. For crabs of the same length of carapace,[2] the frontal breadth seemed to be decreasing.

I have tried to make this clear by a diagram. In Fig. 3 the individuals are classified into twenty-five groups according to length of carapace and the proportional frontal breadth[3] for each class represented for the three years by the position of the circles.[4] The general slope of the connecting lines convinces one that the Plymouth Sound crabs, as observed by Thompson and Weldon, were undergoing a pronounced change in frontal breadth.

The two reasonable hypotheses to account for this decrease are: (1) A modification of the young individuals by the direct action of a changing environment, (2) a decrease in the average frontal breadth in the population due to elimination of the individuals with broader frontal dimensions.

A change in the environmental conditions of Plymouth Sound was undoubtedly in progress during the time when Professor Weldon's observations were made. The streams bring into the sound large quantities

  1. Measurable Characteristics of Plants and Animals), Proc. Roy. Soc. Lond., Vol. XLVII., pp. 360-379, 1894. Also, W. F. E. Weldon, presidential address to the Section of Zoology, British Association, Report of Bristol Meeting (1898), pp. 887-902, 1899. Interesting and valuable supplementary information concerning Weldon 's studies on selective elimination are to be found in Pearson's biographical memoir of Professor Weldon (see Biometrika, Vol. V., pp. 1-52, pl. I.-V., 1906).

  2. The distance between the tips of the extra-orbital teeth, from the point A to the point A' in the figure.
  3. There is no way of knowing precisely how old an individual beast is; if the specimens for different series are sorted into classes of about the same length of carapace, on a line from C to D, and if there is no reason to suspect any differences due to special environmental influences, dimensions of other parts of the shell can be compared in different lots with reasonable confidence that animals of about the same average age are being examined.
  4. The frontal breadth is expressed in thousandths of the carapace length.
  5. For 1898 the number of observations is not large enough for thoroughly satisfactory determinations.