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THE POPULAR SCIENCE MONTHLY.

with any violence, for the claws never snapped nor closed violently; but she was coy, however, and refused to be won by his advances, for the dance may have been nothing new to the lady crab, nor half as interesting as it was to the two spectators outside the water. Later, he too buried himself in the sand, and the performance came to an end.

The next day, and the day following it, the two crabs were watched, but without anything unusual taking place. The colors and markings of the male and female were much the same, though it seemed that the male had slightly more brilliant tints. To determine whether or not there is any marked sexual difference, a greater number of both sexes will have to be examined, and this at the time when the males woo the females under perfectly normal conditions.

Performances such as these are by no means uncommon among the vertebrates, especially with male birds in their endeavors to attract the female; but I believe there are few, if any, performances of this kind on record below the vertebrates.

To any one who has watched the crabs in their natural environments, the complex psychological development which may here be brought into play will not be surprising; yet, if the instinct which leads the male to dance is the same that we see in male birds, and if the female shows any discrimination between the dancers, the mental development must be considerable. Darwin has, in his "Sexual Selection," recorded among Crustacea many instances of difference in structure, and a few cases in which the color[1] of the two sexes is slightly different, but does not mention any performance comparable to the dance of the Platyonychus.

 

ON THE CAUSES OF VARIATION.[2]
By C. V. RILEY, Ph.D., United States Entomologist.

WHATEVER influence we may attach to environment and external conditions, it is self-evident that they alone have not been sufficient to induce the wonderful variety of life existing upon the globe to-day. Indeed, so far as natural selection implies necessary utility, necessary adaptation to surroundings, it is, as I have said, defective. We know very well that introduced species from one continent to another, or from one country to another, have proved better adapted to the changed conditions

  1. Prof. Coun has reported a sexual difference in color in Callinectes.
  2. From the address of the Vice-President of Section F of the American Association for the Advancement of Science, delivered at the Cleveland meeting, August, 1888.