118 



ANALYSIS OF THE ENVIRONMENT 



miles a day. The speed for individual mi- 

 grants or for a given twenty-four hours may 

 be much greater or much less, but the aver- 

 age holds, and this average brings the mean 

 rate of migration within the rule as stated. 

 Northward, the rate of migration is faster, 

 probably because some of the slower spe- 

 cies have stopped to nest, and so the aver- 

 age rate is increased, and because, once 

 started, the season develops faster in north- 

 em latitudes.* 



In China, the bioclimatic rule was fol- 

 lowed, in whole or in part, by eleven 

 species of Lepidoptera (41 per cent of 

 those studied). Three of these species have 

 one annual generation only and overwinter 

 as pupae. Of the sixteen species (59 per 

 cent) that did not conform, six overwin- 

 tered as larvae. The rule seems, in general, 

 to be a useful summarizing statement of a 

 situation that holds for some, though by 

 no means for all, seasonal events. It must 

 be remembered that seasonal changes are 

 aflFected by differences in length of day and 

 frequently by rainfall and other conditions 

 as well as by changes in temperature. 



STRUCTURAL MODIFICATION INDUCED 

 BY TEMPERATURE 



It is easy to produce changes in metabo- 

 lism in response to changes in temperature. 

 Such functional modifications, important as 

 they are at times, are usually reversible and 

 transitory. Those modifications of function 

 that result in phenotypic changes in struc- 

 ture attract more attention because they are 

 both rarer and more obvious. Changes in 

 temperature are known to produce struc- 

 tural modifications, and numerous instances 

 can be cited with the well-studied Droso- 

 phila melanogaster alone, in which, among 

 other structures, temperature affects the 

 number of facets in the eyes, the size of 

 vestigial eyes, and the presence or absence 

 of supernumerary legs (Goldschmidt, 

 1938). 



Cyclomorphosis 



A most striking instance of a relation be- 

 tween body form and seasonal change in 

 temperature is the phenomenon of cyclo- 

 morphosis in some small aquatic organisms, 



" Selected references include Cooke (1917), 

 Hopkins (1918, 1920), Clarke, Margerie, and 

 Marshall (1924), Chapman (1934), and Mell 

 (1935). 



including Cladocera, some simplified as- 

 pects of which are illustrated in Figure 21. 

 The facts as collected from observations in 

 nature are: In Danish waters, at least, a 

 change of form in whole populations of wild 

 Daphnia follows a rise in temperature to 

 between 12° and 16° C. (or to above 19° 

 in Connecticut; Brooks, 1946). The head 

 projections or helmets become fully devel- 

 oped in a few weeks and thereafter remain 

 at their summer size; hence, there is little 



SUMMER 



Fig. 21. Cyclomorphosis in Cladocera, show- 

 ing identical winter forms, but contrasting 

 summer forms. ( Redrawn from Coker. ) 



correlation between the degree of warmth 

 of the water and the size of the helmet. A 

 gradual reversion to the round-headed win- 

 ter form may occur in the autumn; in sum- 

 mer, perhaps after the formation of ephip- 

 pial eggs, the daphnia may disappear to 

 reappear in autumn as ephippial round 

 heads. Correlation of helmets with abun- 

 dance of food, if indeed it exists, is only 

 partial, and all size relations, both with 

 temperature and food, fail when popula- 

 tions from different waters are compared. 

 There is a rough, partial correlation be- 

 tween the size of the body of water and 

 degree of helmet development, with larger 

 helmets in larger bodies of water and their 



