394 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



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 

 color to three pigments — red, yellow, and blue. In the daytime the 

 many changes of color in response to varying surroundings are due 

 entirely to the red and yellow pigments. At nightfall the color of 

 Hippolytc, whatever it may happen to be at the time, changes to a 

 transparent azure blue, this blue color being replaced at daybreak 

 by the prawn's diurnal tint. The iEsop prawn thus exhibits rhythmic 

 color change corresponding to the transition from light to darkness 

 and vice versa. 



So far the cases we have been considering of rhythmical behavior 

 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-established cyclical changes of cli- 

 mate 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 fluctua- 

 tions of climate are believed to have affected the whole world simul- 

 taneously, and it is certain that the whole of the Northern Hemi- 

 sphere was affected. 



The second type of climatic change is less familiar, having been 

 only recently established. Bruckner, Clough, and others consider 

 that the whole world passes through a climatic cycle once in every 

 86 years. At one end of the cycle the climate of continental regions 

 for a period of years is unusually cold and wet, with relatively fre- 

 quent storms ; at the other the climate is warmer and drier, with high 

 barometric pressure and few storms. The extremes of low tempera- 

 ture are ascribed, somewhat paradoxically it may seem, to periods of 

 maximum solar activity as shown by the number of sun spots and the 

 rapidity with which they are formed. 



The Swedish hydrographer, Petersson, has published data show- 

 ing the importance of these climatic cycles to the study of hydro- 



