652 OYSTERS, AND ALL ABOUT THEM. 



temperature of the water hinder their extension, for the 

 variation is the same over the oyster-beds as at other 

 points, and it fluctuates, during the course of the year, 

 from 20 C. above zero to 20 C. below. Nor can a lack 

 of motion of the water, or of nutriment, be the cause why 

 the oyster-beds have not, during the past hundreds of 

 years, extended themselves beyond certain definite limits, 

 for floating everywhere, in the ebbing and flooding water, 

 are microscopic plants and animals, and much dead organic 

 matter, which would nourish large numbers of oysters, just 

 as they do multitudes of soft clams (mya arenariaj, edible 

 mussels (mytilus edulisj, and cockles (cardium edulej. 

 There remains, then, as the single natural hindrance to a 

 further extension of the oyster-beds, the unfavourable con- 

 dition of the ground over the greater portion of the sea- 

 flats. Oysters cannot thrive where the ground is composed 

 of moving sand, or where mud is being deposited, and one 

 of these conditions or the other is found over the greater 

 part of the sea-flats. The number and size of those places 

 where, notwithstanding the daily ebb and flood currents, 

 the ground remains unchanged and free from mud, are very 

 limited. Only along the slopes of certain channels to the 

 north of the mouth of the Eider do we find united all the 

 conditions favourable for such places, and only within 

 these limited districts can young oysters grow to complete 

 maturity. 



CAN THE FRENCH SYSTEM OF ARTIFICIAL OYSTER- 

 BREEDING BE CARRIED ON IN THE WATERS OF THE 



GERMAN COAST ? 



. . . . As regards the saltness of the water, the 

 currents, the food, and even the composition of the soil, 

 our sea-flats will compare as favourably for the artificial 



