C. L. PROSSER 355 



breeding were possible only for those oysters brought into the 

 laboratory during the winter (Davis, 1955). Similarly, the oyster 

 drills, Urosalpinx cinerea of Virginia and Delaware, have differ- 

 ent temperature thresholds for spawning (Stauber, 1950). The 

 effect of latitude on rate of growth of embryos of various marine 

 invertebrates has been well summarized by Delmel (1955) who 

 added observations on certain species of snails from southern 

 California and Alaska. When growth rate was measured at 

 different temperatures in the laboratory, the Alaskan snails grew 

 significantly faster at corresponding stages ( Fig. 4 ) . In the middle 

 of the physiological temperature range the northern Qi values 

 were higher in those snails which showed no overlap in rates, 

 lower in the others. Similar findings have been reported earlier 

 for European marine invertebrates (Fox, 1938; Thorson, 1951). 

 These differential growth rates can partly account for the rela- 

 tively greater productivity of colder waters. It appears that the 

 Alaskan and southern Californian populations of the same species 

 of snails are genetically different, yet no transplantation experi- 

 ments were performed and the animals at intermediate latitudes 

 were not compared. It is likely that clines with respect to the 

 effect of temperature on development of marine invertebrates 

 may occur and that between the ends of such clines major 

 genetic differences exist. However, there is need for transplanta- 

 tion of larval stages and acclimation over a life cycle. 



Among insects, several clines have been described where 

 temperature seems to be critical for development. In the moth 

 Lymantria dispar, Goldschmidt (1940) showed in his classic 

 studies that the rate of development in south European races is 

 slower than in north European populations. In pine-needle wasps 

 (Diprion) alpine races show faster development in the egg and 

 lower Qios, especially for conversion from pupa to adult, than 

 do populations from warmer regions (Elens, 1953). 



Rana pipiens from northern United States breed at lower tem- 

 peratures, develop faster, have lower temperature coefficients of 

 development than frogs from extreme southern states and low- 

 lands of Mexico (Moore, 1949). Slight morphological differences 

 and a zone of hybridization confirm the conclusion that the 



