232 THE EFFECT OF TEMPERATURE 



are extremely thin. " These remarkable variations are, 

 no doubt, to a large extent due to the violent changes 

 of temperature which are experienced in the Baltic, 

 and by which the steady development of the animals in 

 question is thrown out of gear. The same species occur 

 on the coast of Greenland and Iceland, where they at- 

 tain a considerably larger size than in the Baltic, in 

 spite of the lower mean temperature, probably because 

 their development is not interrupted by any sudden 

 change from cold to heat, or vice-versa."* 



The influence of a low temperature on the colour of 

 marine Mollusca seems to be indicated by the observa- 

 tions of Fischer f on the shells of the west coast of 

 South America. Numerous species of these shells ex- 

 hibit a remarkable degree of melanism, and it seems 

 highly probable that " this concurrence of specific 

 melanism (which stands quite alone in the world) is due 

 to the cold polar current which impinges on the Chilian 

 coasts, for the same genera occur on the opposite shores 

 of the continent without exhibiting any trace of this 

 mournful characteristic." $ It is very improbable, 

 however, that this melanism is the direct result of the 

 cold current. If so, why should it not be observed in 

 other parts of the world, which are similarly visited by 

 cold currents? 



More interesting and unequivocal effects of tempera- 

 ture are afforded by the numerous experiments which 

 have been made upon the wing colours and markings of 



* Quoted from Cooke, "Cambridge Natural History," vol. iii. 

 p. 84. 



f Journ. de Conchy!. , xxiii. p. 105, 1875. 

 \L. c., p. 85 



