Ill] THE EFFECT OF TEMPERATURE 219 



no fruit in Judaea, and the vine bears freely at Eshcol, but not in 

 the hotter country to the south*. Shellfish have their own appro- 

 priate spawning-temperatures; it needs a warm summer for the 

 oyster to shed her spat, and Hippopus and Tridacna, the great clams 

 of the coral-reefs, only do so when the water has reached the high 

 temperature of 30° C. For brown trout, 6° C. is found to be a 

 critical temperature, a minimum short of which they do not grow 

 at all; it follows that in a Highland burn their growth is at a 

 standstill for fully half the yearf. 



That a rise of temperature accelerates growth is but part of the 

 story, and is not always true. Several insects, experimentally 

 reared, have been found to diminish in size as the temperature 

 increased J; and certain flies have been found to be larger in their 

 winter than their summer broods. The common copepod, Calmius 

 finmarchicus, has spring, summer and autumn broods, which (at 

 Plymouth §) are large, middle-sized and small; but the large spring 

 brood are hatched and reared in the cold "winter" water, and the 

 small autumn-winter brood in the warmest water of the year. In 

 the cold waters of Barents Sea Calanus grows larger still; of an 

 allied genus, a large species lives in the Antarctic, a small one in 

 the tropics, a middle-sized is common in the temperate oceans. 

 The large size of many Arctic animals, coelenterates and crustaceans, 

 is well known; and so is that of many tropical forms, Hke Fungia 

 among the corals, or the great Tritons and Tridacnas among 

 molluscs. Another common phenomenon is the increasing number 

 of males in late summer and autumn, as in the Rotifers and in the 

 above-mentioned Calani. All these things seem somehow related 

 to temperature; but other physical conditions enter iilto the case, 

 for instance the amount of dissolved oxygen in the cold waters, and 

 the physical chemistry of carbonate of Hme in the warm||. 



The vast profusion of life, both great and small, in Arctic seas, the multitude 

 of individuals and the unusual size to which many species grow, has been 

 often ascribed to a superabundance of dissolved oxygen, but oxygen alone 

 would not go far. The nutrient salts, nitrates and phosphates, are the 



* Cf. J. W. Gregory, in Geogr. Journ. 1914, and Journ. E. Geogr. Soc. Oct. 1930. 



t Cf. C. A. Wingfield, op. cit. supra, p. 176. 



X B. P. Uvarow, Trans. Ent. Soc. Lond. lxxix, p. 38, 1931. 



§ W. H. Golightly and LI. Lloyd, in Nature, July 22, 1939. 



li Cf. B. G. Bogorow and others, in the Journ. M.B.A. xix, 1933-34. 



