1S4 CARNEGIE INSTITUTION OF WASHINGTON. 



than those they replace in reducing the visibility of inactive fishes, the facts 

 noted, and others, suggest very plainly that existing color-patterns in animals, 

 no less than their colors themselves, are not a product of haphazard, uncon- 

 trolled mutation; for it is improbable that fixed colors and patterns are with- 

 out biological significance, when changeable ones commonly vary according 

 to a definite system. 



The observations upon Brachyura refer to the prevalence of color changes 

 within the group, the causes which determine them, and the conditions under 

 which they occur in nature. 



With respect to the first point, it may be inferred that changes in color do 

 not occur uncommonly within the limits of the suborder, although they have 

 apparently been reported in only two species. To these, Gelasimus sp. and 

 Ocypoda arenaria, may be added Portunus depressifrons, P. sayi, P. spini- 

 carpus, Callinedes ornatus, C. marginatus, and Euryplax nitida. 



It seems clear, in addition, that both in the laboratory and in nature the 

 fundamental factor in determining the shade of exposed specimens is that of 

 the substratum on which they stand. This is at variance with the conclusion 

 deduced from his study of Ocypoda by Dr. R. P. Cowles,^ but is indicated by 

 the following facts : 



In the subdued light of the laboratory, at a temperature of 30° C, for 

 example, individuals of the species mentioned above, with the exception 

 of the Brazilian Gelasimus, become dark or light respectively in uncovered 

 black or white dishes of sea-water. In addition, specimens of Ocypoda, the 

 form experimented upon most extensively, having stood long enough to be 

 adapted in color to their situation, may be cooled in white dishes to 12° C. 

 or lower or warmed in dark ones to approximately 35° C. without changing 

 in coloration under the treatment. Again, if crabs are placed in black and in 

 white dishes of sea-water and permitted to adjust their coloration to their 

 surroundings, after the light one has been cooled and the other warmed, each 

 in its own vessel, upon being transposed they will readjust themselves and 

 reverse their original relation in the face of a temperature difference which 

 should make this impossible if temperature were the basic factor in determining 

 their shade. 



The temperature difference involved in the last experunent may be so great 

 that there can be little doubt of the significance of the result. In one instance, 

 while the double change was being accompUshed, the water in the cooler dish 

 neither rose above 17° C. nor fell below 35° C. in the other. The time required 

 for the two crabs to attain the same shade in this case was 29 niinutes and the 

 reversal of the original relation was completed in about 50 minutes. These 

 records, however, convey a false idea of the rate at which such changes com- 

 monly occur, for the water in the warmer vessel was for a time above the 

 temperature at which a true heat-blanching is induced. Half or even one- 

 third of the tune indicated commonly suffices for the making of such adjust- 

 ments in young specimens whose changes are most evident. 



In Callin9ctes marginatus, at least, such adaptive changes in shade as have 

 been noted occur in nature. When one walks in the shallow water along 

 beaches where this species is found, the individuals that dart away from under 

 foot are dark or light, according as they spring up from dark or light patches. 

 It is to be anticipated that similar adjustments are made by the other change- 

 able species. 



^Cowles, R. P.: Habits, reactions, and associations in Ocypoda arenaria. Papers from the 

 Tortugas Laboratory, Carnegie Inst. Wash. Pub. No. 103, pp. 1-41, 1908. 



