1901.] on Colour in Amphibia. 593 



A very common pattern of Frogs and Tree-frogs is that of longi- 

 tudinal stripes. One stripe, generally light, runs along the middle 

 of the back ; others, in pairs, extend from the head along the sides of 

 the body and converge backwards. Others again, more irregular, 

 begin on the flanks and run across the folded limbs. There is only 

 one median line, and this is in a unique position and condition so 

 far as nerve- and blood-supply are concerned. This median stripe 

 makes a break in the look of the whole upper surface, thereby 

 rendering the latter less conspicuous. The other stripes are ruled 

 by the law of symmetry ; right and left are counterparts. We have 

 only to look at a Frog in its typical attitude when it sits in the grass. 

 We then see at once that the shado as and lights of the green and 

 brown stalks produce a pattern strikingly like that of the European 

 Water-frog and the Australian Hyla aurea. 



We may safely assume that such stimulative colouring has been 

 in many cases the cause, the origin of the permanent and hereditary 

 adaptive colouring. For when a species has lived for a long time 

 under conditions which over and over again cause the chromato- 

 phores to be influenced in a particular way, causing an ever-repeated 

 arrangement, then this pattern may at last fix itself on the animal. 

 At first, by combined repetition, certain parts of the skin will be 

 coached to react in the same way — a kind of predilection, later on a 

 habit will set in, until this fixes itself unchangeably. 



Of course we do not assume that our Frog has always been sitting 

 under the same blades of grass, nor that the shadows fall upon the 

 same spots. But grass is grass, and the general effects of its distri- 

 bution of light and shade are the same, and all we assert is that the 

 outcome of this stimulus will be a marshalling or exciting of the 

 pigments in a linear direction. Nowadays the whole process is 

 infinitely more complex. Our suggestion aj>plies to the primordial 

 frog, of which, however, we know nothing, except that it existed and 

 must have acquired its colours and patterns at some time or other. 

 Those individuals which we can watch now, start already with the 

 inherited capacity of developing that pattern which their ancestors 

 have managed to acquire during countless generations. This 

 individual need not sit in the grass and wait for the colours and 

 shadows to photograph themselves upon its back, and yet this same 

 process is still going on. If our Frog did not live in a grassy 

 country, but in other surroundings, it is absolutely certain that he, 

 and to a greater extent his offspring, would develop into a new 

 colour-variety. The common frog which lives on granitic soil 

 assumes and keeps the speckled colour of the ground, and looks very 

 different from those members of its kind which live on dark moor- 

 lands, or among rich vegetation. And these colour varieties have 

 not the same range of changes, since their very pattern has been 

 altered. 



There are, in fact, many instances of what seems to support the 

 idea of natural photography. This term has rightly been objected 



Vol. XVI. (No. 95.) 2 a 



