Introduction 



place or found along a dusty road are light in colour, while those 

 kept or found in moist surroundings will prove to have the pig- 

 ment cells spread so that the skin is dark-coloured. Activity 

 on the part of the toad or frog has a marked tendency to pro- 

 duce a change to lighter colours; this is true even when there 

 are counteracting influences, such as darkness. 



The change from light to dark and vice versa may take place 

 in as short a period of time as ten minutes (Hyla regilla, Hyla 

 squirella, etc). Again, the time necessary is habitually a half-hour 

 or more, as is the case with Hyla versicolor and Hyla gratiosa. 

 The rapidity varies with the species; but this further fact is al- 

 ways true, that in all species colour changes are likely to take 

 place more rapidly during the season of greatest activity (the 

 breeding season) and at a time when the creatures are abundantly 

 fed. During great scarcity of food, when the vitality is reduced, 

 the frog assumes a light colour, and retains such even under 

 strong counteracting influences. 



Notwithstanding all these facts, there must be much not yet 

 understood regarding the change from light to dark and the re- 

 verse, since there are so many contradictions of these facts that 

 come under observation. Almost any given Hyla (gratiosa, versi- 

 color, cinerea, squirella, regilla) may take on some given colour, 

 usually a medium shade, and retain it for months without any 

 change during bright days and dull, when warm or cold, wet or 

 dry. This is true, also, of various species of Rana, perhaps of 

 all ; they seem to have a natural individual tendency toward a given 

 colour or shade and retain it in spite of changing conditions and 

 in company with other individuals of the same species which con- 

 stantly respond to these changes. 



Again, frogs and tree frogs sometimes surprise one by be- 

 coming their lightest when kept in complete darkness and when 

 all conditions would seem to point towards their taking on their 

 dark dress. This is noticeably true of Hyla pickeringii and Hyla 

 cinerea. Again, some species of tree frogs, Hyla squirella in par- 

 ticular, will change back and forth again and again very rapidly 

 for hours at a time when the conditions remain the same. Thus 

 the process of change would seem not to be a wholly automatic 

 one nor the result entirely of reflex action. There are probably 

 internal factors to be taken into account, factors that have to 



