484 THE POPULAR SCIENCE MONTHLY. 



with any violence, for the claws never snapped nor closed vio- 

 lently ; 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 per- 

 formance 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 de- 

 termine 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 nor- 

 mal conditions, j 



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, perform- 

 ances of this kind on record below the vertebrates. 



To any one who has watched the crabs in their natural envi- 

 ronments, 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 * 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.f 



By C. V. EILEY, 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 exist- 

 ing 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 intro- 

 duced species from one continent to another, or from one country 

 to another, have proved better adapted to the changed conditions 



* Prof. Coun has reported a sexual difference in color in Callinectcs. 

 j- 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, 18Ss. 



