ARTICLES 423 



of course, not permanent, but becomes gradually weaker with 

 the passage of time. Numerous similar cases occur, both in 

 plants and animals, but the above are sufficient for our purpose. 



It is common knowledge that in many animals the colour 

 is not fixed, but varies according to the hue of their surround- 

 ings. This power of colour change has been investigated 

 most minutely in the case of the iEsop prawn (Hippolyte 

 varians). 1 The colour of this crustacean is extremely variable, 

 the change in coloration being brought about by the expansion 

 or contraction of masses of pigment. The cells in which the 

 pigment is situated are very irregular in form, with branched 

 processes. On appropriate stimulation, the pigment flows out 

 along these branching spaces in such a way that what was 

 a mere pin-point of pigment becomes spread out over a wide 

 surface. The result is a change in coloration. The iEsop 

 prawn owes its colour to three pigments — red, yellow, and 

 blue. In the daytime, the many changes of colour, in response 

 to varying surroundings, are due entirely to the red and yellow 

 pigments. At nightfall the colour of Hippolyte, whatever it 

 may happen to be at the time, changes to a transparent azure 

 blue, this blue colour being replaced at daybreak by the 

 prawn's diurnal tint. The ^Esop prawn thus exhibits rhythmic 

 colour-change corresponding to the transition from light to 

 darkness and vice versa. 



So far the cases we have been considering of rhythmical 

 behaviour are all of a clear-cut and simple kind, being directly 

 due to the succession of the seasons, the regular alternation 

 of night and day, the ebb and flow of the tides, etc. ; moreover, 

 the nature of these latter phenomena is also well understood. 

 On the other hand, the causes underlying the now well-estab- 

 lished cyclical changes of climate are not only themselves 

 decidedly more complex, but there is still a great deal to be 

 learned about their effects in relation both to animals and 

 mankind. 



The first type of climatic change, and the best known, is 

 that of the Glacial Period. It is now known that the Glacial 

 Period was not a continuous period of intense cold, but was 

 punctuated by epochs in which the weather was much warmer, 

 and when the retreat of the ice-sheet allowed many animals 

 and plants to regain, temporarily at any rate, the ground they 

 had been obliged to cede. These fluctuations of climate are 

 believed to have affected the whole world simultaneously, and 

 it is certain that the whole of the Northern Hemisphere was 

 affected. 



The second type of climatic change is less familiar, having 



1 Keeble, F. and Gamble, F. W\, Phil. Trans. Roy. Soc, B, vol. cxcvi. (London, 

 1904.) 



