The Pacific Field 273 



variation in temperature during the year. In the 

 Atlantic, a set of spat is rarely obtained if the water tem- 

 perature often falls much below 70° F. during the breed- 

 ing season. Failure of the set is common in the north- 

 ern field, probably because of rapid declines in tempera- 

 ture from cold summer rains. 



On the Pacific coast, even in the bays, the maximum 

 summer temperature of the water rarely exceeds 70°, and 

 at most places the average is nearer 6o°. It is not to be 

 expected, then, that eastern oysters may reproduce in 

 these waters. But in some of the creek mouths of San 

 Francisco Bay, the shallow water is kept warm by the 

 sun, and it is in them that reproduction has been ob- 

 served. 



Perhaps it is possible that the species might be ac- 

 climatized to a lower temperature, but this could be ef- 

 fected only in several generations. There is no reason 

 to expect that any constitutional change that might ac- 

 custom any individual oyster to its new surroundings, 

 would restore its lost fertility. But it may be that the 

 species could again be made fertile by following the 

 selection method of breeders of domesticated animals 

 and plants. After careful observation of summer tem- 

 perature, bottoms might be selected over which the mini- 

 mum was somewhat below 70°. If large numbers of 

 eastern oysters were planted here, some individuals might 

 possibly be found to possess the constitutional power of 

 breeding at a slightly lower temperature than that re- 

 quired by the majority. Whether this is true or not in 

 this case has never been observed, but similar individual 

 differences have been found and taken advantage of in 

 the cases of many other organisms. 



If now some individuals of the first generation should 



