480 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



coloration may be apprehended; for it is to that environment that 

 it is adapted. 



It is a simple proposition, and should be self-evident, that, al- 

 though two species may often be seen doing the same things to- 

 gether, neither their distribution nor the range of their activities 

 is therefore of necessity identical. Nevertheless, writers overlook 

 this fact, and reason unsoundly that because two or more species 

 occur together, pursuing their occupations "cheek by jowl," and 

 yet differ in pigmentation, both can not be obliteratively colored. 



Painters and plumbers, capitalists and coal heavers, may at times 

 be seen together similarly engaged, yet their differences in dress and 

 demeanor are not without practical relation to their respective 

 callings. No one forgets that they are not always rubbing shoulders 

 reading the same newspaper bulletins, or paying the same income 

 taxes. Similarly, although the colors displayed by mixed swarms of 

 fishes upon the reef are varied, although some are matched by noth- 

 ing one sees in their vicinity and the patterns in which they appear 

 may be highly contrastive, each particular combination may serve, 

 and serve in essentially the same way, under the conditions in which 

 it is most commonly to be seen. The truth will appear, however, only 

 when the different species have been observed under many conditions, 

 and the range and activities of each have been defined ; until which 

 time no one's suggestion of the revealing or concealing effect of their 

 colors and patterns may claim to rank higher than an interesting 

 working hypothesis. 



In determining w T hen and where a given species is to be found, 

 and how it spends its time, which are the fundamental facts con- 

 cerning it in the present connection, no single method is more valu- 

 able than the analysis of stomach contents of individuals taken from 

 as many and varied localities as possible, and at all times of day 

 and night when they may be secured. 



Application of this principle shows that there is, on the whole, a 

 sharp distinction between diurnal and nocturnal species, since com- 

 paratively few seek and capture food indifferently at all hours. To 

 which group a form belongs is usually clearly indicated by the 

 amount and the stage in digestion attained by the food in the fishes' 

 alimentary tract morning and evening. In some species every indi- 

 vidual of hundreds examined will be full at daylight and fasting at 

 sunset. Species of which this is true commonly school at definite 

 points during the day. 



The fishes in figure 2, plate 1, are in characteristic grouping in 

 typical surroundings. Such small knots, and larger aggregations of 

 nocturnal feeders, are semipermanent in composition ; for individuals 

 which may be distinguished by some peculiarity, such as size, wounds, 

 scars, or malformations, may be seen clay after day at the same place. 



