1036 OYSTERS, AND ALL ABOUT THEM. 



course, that the superior nutrition of the banks may have an 

 influence in expediting the ripeness of the spawn which is 

 missed when the arrival on the banks is delayed. 



The process of incubation is much affected by tem- 

 perature. The period required for hatching herring ova 

 varies from a week, when the temperature is 54, to six 

 weeks when it is 38. In the case of cod, a range of from 

 thirteen to fifty days has been observed, and so on with 

 other fishes. 



Lastly, there are the effects of temperature on the rela- 

 tive scarcity or abundance of food. In this branch of the 

 investigation little progress has yet been made. There is 

 reason to suspect considerable differences from year to year 

 in the supply of surface food entomostraceans, crusta- 

 ceans, &c. corresponding to the differences in the swarms 

 of locusts, flies, and other insects on land. The precise 

 effects of heat and cold on the production of the different 

 marine invertebrates, have still to be determined. The 

 ova of molluscs and annelids haye been found in the 

 Baltic at all seasons of the year, but seasonal variations 

 and intermissions of the reproductive process seem to be 

 the all but universal rule, and there is at least probability 

 in the remark of Prof. Mobius, that " the periodical in- 

 crease and decrease of nutritive matter in the different parts 

 of the sea depend on the degree of warmth and light which 

 during the changing seasons they receive from the sun." (h) 



In their thoroughly practical and excellent pamphlet 

 on " Oyster Culture," (from which I gratefully acknow- 

 ledge having quoted largely in the latter portion of this 

 work,) Messrs. Anson and Willett evidently disagree with 

 the foregoing authority on the effects of temperature, and 

 give the following reason :- 



(7i) Ibid. 



