YERKES. VARIATION IN THE FIDDLER CRAB. 



421 



marks it was possible to read to tenths. Although in case of these 

 measurements readings were made to tenths, accuracy to fifths is all that 

 can be claimed, because of the tendency to judge in favor of the marks 

 on the scale. 



Over a thousand individuals of Gelasimus pugilator were collected, 

 without any attempt at selection, from a large colony on the shore of 

 West Falmouth Harbor, between July 

 24th and August 22d. The accompanying 

 map of the harbor and vicinity shows the 

 location of the colony ; two small circles 

 mark the limits of the ground used for 

 collecting. 



Weak alcohol, about 20%, gave satis- 

 factory results as a killing agent. Sub- 

 stances which act very quickly, such as 

 hot water, formol, or strong alcohol, 

 cause autotomy. 



Twelve measurements were made for 

 each of four hundred right-handed ani- 

 mals, and for the same number of 

 left-handed individuals. For these two 

 groups of measurements the means, 

 modes, standard deviations, and coeffi- 

 cients of variability have been found. 



To avoid the possibility of misunder- 

 standing as to the meanings of these 

 terms, I shall briefly define them.* The 

 Mean is the sum of the products of each 

 class into the number of individuals in the 

 class, divided by the total number of individuals. The Mode is the class 

 containing the greatest number of individuals. The Standard Deviation 

 "is found by adding the products of the squared deviation-from-the-mean 

 of each class multiplied by its frequency, dividing by the total number 

 of variates, and extracting the square root of the quotient, "f The 

 Coefficient of Variability is one hundred times the quotient of the standard 

 deviation divided by the mean. 



Figure 3. Map of West Fal- 

 mouth Harbor. Material was 

 collected from the region between 

 the small circles. 



* The terms and methods given by Davenport ('99"). 

 t Davenport ('99", p. 15). 



