592 Mr. Ham Oadow [April 26, 



direct influence of the surroundings in which a particular frog hap- 

 pens to live, be these factors light, temperature or food, or all three 

 combined, will probably affect or change the pigmentation in certain 

 ways, perhaps at first to a very small extent, almost imperceptibly. 

 It stands to reason that the offspring, living under similar conditions, 

 will be acted upon in the same way. That factor, for instance, which 

 has added green to the parents will add green to the children, until 

 by cumulative inheritance a more decidedly green race is produced. 

 And then, but not before, natural selection can step in. Then is the 

 time to decide if the new colour is useful or not. If harmful, that 

 race will soon come to grief; if advantageous, in ways hitherto not 

 dreamed of, it will probably increase. In fact we have to resort to 

 the direct influence of the surroundings, and this implies many fac- 

 tors. How can it otherwise be explained that the same kind of 

 pattern, the same combination of colours, occurs not only in members 

 of the most different groups of Newts, Toads and Frogs, but also in 

 widely separated countries ; in other words, in creatures which, we 

 feel absolutely certain, had nothing to do with each other, except 

 that they all are Amphibia ? 



Such instances of parallel or convergent development supply the 

 strongest argument for assuming that it is the direct influence of the 

 surroundings which models, in our case colours, the creatures, instead 

 of working upon the basis of freaks. The latter idea implies an end- 

 less uninterrupted chain of accidental, spontaneous variation, always 

 in the same direction. A single break in the chain, and the whole 

 process would have to begin again. Black, red and yellow are due 

 to corresponding pigments. It is possible that warmth and light, 

 the distribution of the blood-vessels, unknown ingredients of food, 

 have caused them to be deposited in the skin. We may even assume 

 that the guanine crystals, which are certainly a product of the meta- 

 bolism of the body, and which belong to the uric acid group, have, 

 through the action of the light, been deposited in the skin, thus 

 accounting for blue, green and white. These crystals and the black 

 pigments are in reality noxious waste products, which, instead of 

 being got rid of in the ordinary way, have stuck in the system, but 

 deposited where they cannot do any harm. If their accumulation in 

 the skin, by producing visible colour, happens to turn out useful, all 

 the better, and then we may be sure that natural selection will be busy. 



Now as to pattern of colour. The chromatophores are very 

 susceptible to temperature and light, besides being controlled by 

 the nervous system. Higher temperature and bright sunlight cause 

 them to contract, and the respective parts become paler. Lower 

 temperature and absence of sunlight dilate them. These very factors 

 are actually at work whenever a Frog sits in sunlight and vegetation. 

 There is light and shade. Some Tree-frogs — e.g. Hyla hypochondri- 

 alis, of South America, have such a delicately sensitive skin that the 

 shadow of a blade of grass causes a rapid darkening of the shadowed 

 part of the skin. 



