14 PROCEEDINGS OP THE AMERICAN ACADEMY 



has, of course, a direct bearing upon the question of mimicry ; but it 

 must be frankly stated that, as far as the causes of coloration among ani- 

 mals have been studied, it is difficult to see how natural selection can 

 have been a factor in producing permanent mimicry ; while the rapid- 

 ity with which many fishes adapt themselves to the color of the bottom 

 upon which they live enables them undoubtedly to produce a protective 

 coloration, which is of advantage to them ; and constant habit may 

 develop unequally the capacity of producing certain tints, or patterns 

 even, which in their turn may be transmitted, and thus readily account 

 for the lighter coloring of Flounders living upon sandy bottoms, as com- 

 pared with those living upon rocky bottoms covered with dark algae. 

 Yet place the latter upon a light ground and the former upon a dark 

 ground, and they will very soon adopt the proper coloration of their 

 bottom, showing they have not lost their power of changing. As for 

 many of the patterns of coloration of birds and in insects, produced 

 by physical causes, it seems quite impossible to look upon them as the 

 fortuitous product of the action of light, or to regard it as an efficient 

 cause of protective mimicry. 



The pigment cells appear early in the egg. In some of the fishes, we 

 have even two color elements in the older stages, immediately before 

 the young fish is hatched, — viz., the black and yellow ; but, in the 

 majority of cases, the black alone is present, the yellow element ap- 

 pearing subsequently, and, last of all, the red. The experiments made 

 by Pouchet on pigment elements show that the blue pigments are 

 probably only a dimorphic condition of the red pigments. This 

 would give a ready explanation why Lobsters turn red when cooked, 

 and of the blue Lobsters which are occasionally caught. The same 

 may also be said of green. Violet pigment, which is found in some 

 Crustacea, gives special reactions. 



The anatomical elements containing the pigment are greatly changed 

 during growth. The examination of the pigment spots of the young- 

 est fish on any of the Plates here given with more advanced stages 

 shows how great is the capacity for expansion in the black pigment ele- 

 ments, which from mere dots have almost become special organs capable 

 of great expansion and contraction. Pouchet calls the pigment ele- 

 ments chromatoblasts in their embryonic condition, to distinguish them 

 from the chromatophores into which they eventually develop. In 

 addition to the chromatoblasts and chromatophores, Pouchet has also 

 called attention to a third set of bodies, which he calls iridocytes. These 

 are found in Fishes, Reptiles, Mollusks : they are situated near the 

 surface of the integument, and produce the phenomena of iridescence 



