540 W. H. LONGLEY 



consequently before the female abdomen has been exposed in 

 any attitude differing from that assumed by the male. With the 

 onset of reproductive activity the pleon is elevated and covers 

 the egg mass which the female bears. It then continues the 

 dark color of the thorax posteriorly down over it into the shadow 

 with delicate countershading. 



The point to be established is of such importance, that the 

 reason for believing that color differentiation precedes sexual 

 maturity, i.e., that difference in coloration antedates difference 

 in exposure, must be stated in full. The facts are as follows : 



When a large collection of the crabs is made the two sexes are 

 taken in approximately equal numbers. The males are all of 

 one sort differing only in size, but the females are of two types. 

 The larger ones, some of which may be bearing eggs, fall into 

 one group. The others ranging from medium to smallest sizes, 

 are intermediate between males and adult females in several 

 respects. The abdominal margins of such specimens are straight 

 instead of convex, and the contour of the abdomen is therefore 

 triangular rather than broadly rounded. The groove in the 

 sternum into which the pleon fits is relatively deeper and retains 

 it more firmly than in females of the other type. Finally, the 

 openings of the oviducts are so small as to be practically obsolete, 

 though they are conspicuous in mature specimens of the same 

 size. That these are not abnormal individuals appears from the 

 fact that every female apparently passes through this stage, 

 as all the smaller ones show these characters. 



If then there are animals, such as these, whose countershading 

 is a congenital character, and others on which the same gradation 

 of pigment appears, with possibility of the same explanation, 

 one risks much who asserts with assurance, that the condition 

 observed in the second group is due to causes entirely different 

 from those operative within the fii'st. That it is not clear in 

 what way the countershading originated and whether it is, or has 

 been preserved by natural selection, will not warrant such action. 

 The observations upon crabs, therefore, justify the rejection of 

 insolation as a universal immediate cause of development of 

 dark pigment upon animals' surfaces that are habitually turned 



