MACKERELS. 131 



still ; as they are said to appear among the 

 Orkney Islands, and in the Frith of Forth about 

 the end of July or the beginning of August. 



It is now generally believed that the whole of 

 the phenomena of the seasonal appearance and 

 disappearance of fishes may be accounted for on 

 the principle, now pretty well ascertained, that 

 the vivification of the spawn requires its deposi- 

 tion in situations where the sun's rays can have 

 ready access to it. But this would be impos- 

 sible if it were deposited on the bottom in con- 

 siderable depths of water ; and hence, these ani- 

 mals have been endowed with instincts, which 

 impel them at the proper period, to resort to the 

 shallows of the coast, where the incumbent stra- 

 tum of water is not too great to allow the solar 

 light and heat to penetrate to the sand and 

 gravel of the bottom, among which the ova are to 

 find their resting-place. 



On this interesting subject we are glad to 

 quote the opinions of one of the most illustrious 

 of ichthyologists. *' It does not appear," ob- 

 serves Mr. Yarrell, " to have been sufficiently 

 considered, that, inhabiting a medium, which 

 varied but little either in its temperature or pro- 

 ductions, locally, — fishes are removed beyond the 

 influence of the two principal causes which make 

 a temporary change of situation necessary. In- 

 dependently of the difficulty of tracing the course 

 pursued through so vast an expanse of water, the 

 order of the appearance of the fish at different 

 places on the shores of the temperate and south- 

 ern parts of Europe is the reverse of that which, 

 according to the theory [of the older naturalists], 

 ought to have happened. It is known that this 



