OF LIFE IN GENERAL. 



319 



action of a more or less correspondingly variable en- 

 vironment. It follows, therefore, that the environ- 

 ment at Plymouth and at Helder is more variable than 

 that at RoscofT and at Sheerness. 



To turn from marine Invertebrates to marine Verte- 

 brates, the local races of the Mackerel have recently 

 been studied in considerable detail by Garstang.* 

 Some of the chief of the important results obtained by 

 him are embodied in the accompanying table : 



The number of black transverse bars or stripes across 

 the sides of the fish was found to vary from 23 to 33, 

 27 being in almost every instance the most frequently 

 occurring number. The differences in the numbers of 

 bars occurring in the various samples do not seem very 

 great, but it is noticeable that all the samples from the 

 North Sea and English Channel had invariably less 

 than 27 bars (on an average), whilst those from the 

 coasts of Ireland and America had more than 27. On 



* Journ. Marine Biol. Soc., vol. v. p. 235, 1898. 



