CONTROL BY THE EYE 527 



make : the quick changes he called 'physiological', the slower ones, requir- 

 ing weeks or months and having their basis in an increase of the amount 

 of pigment or the number of pigment cells, or both, he called 'morpho- 

 logical' color changes. These terms are not too good, for both kinds of 

 change are equally physiological phenomena; but they have stuck. The 

 very existence of morphological changes was questioned by reviewers right 

 up to 1928, but in the past decade evidence for them has been piled up. 



In 1910-1913 Babak, working with salamander {Amby stoma) larvae, 

 came to a conclusion which is now known as Babak's law: If the con- 

 ditions for producing a given physiological color change are maintained 

 for a long period, the corresponding morphological change will take 

 place if it is within the capacity of the animal. Modern experiments, 

 especially those of Francis Sumner and his co-workers at the Scripps 

 Institution of Oceanography, tend to show that while Babak's law holds 

 pretty well, the relationship it expresses is not a genetic one. Morph- 

 ological changes are apparently not the direct result of the chromato- 

 phoral system's setting itself in a given state and holding that state — 

 rather, the two kinds of changes have a common cause. 

 Control Through the Eye — This cause is always an intricate one, and 

 varies from group to group of animals. Lister established in 1858 that in 

 the frog the eye initiates the process of dermal change, and we now know 

 that this is nearly always true. If the eyes of poikilochromic (/. e., color- 

 changing) vertebrates are covered or removed, no further responses to 

 background — or at most only slight ones — occur. Responses to temper- 

 ature, and to light and darkness, may however go on about as before. 



The eye is thus not only the receptor for vision, and for a host of 

 reflexes concerned with its own control, but it also mediates a reflex arc 

 of some sort which ends in the dermal chromatophores. What constitutes 

 the middle of the arc- — whether nerve impulses or blood-borne substances 

 — is another matter. Before considering that matter, it needs pointing 

 out that for the eye to control dermal responses to its field of reception 

 has no implications whatever for vision in that field. We need not sup- 

 pose that for an animal to respond to a background, he must be visually 

 conscious of its characteristics of hue and tone. As a matter of fact, the 

 eye of a fish can adjust its melanophores to different neutral backgrounds 

 whose difference in tone is too small for the same fish to discriminate 

 visually in a training procedure! Of course in the work of Mast cited in 

 the preceding Section, the instant choice of a particular background by a 

 flounder adapted, dermally, to that background certainly had a basis in 



