312 THE SAND-EEL FAMILY. 



In this, the larvae from 4'5 mm. to 10 mm. are shown to 

 inhabit the bottom (B), and after this length is attained to move 

 upward through the mid-water (M) till at 15 to 20 mm. and 

 over they are found at the surface ($). The curves have been 

 determined by the measurement of preserved specimens so that 

 the rate of growth is also indicated. 



A further inspection of the diagram will show that in July 

 and the earlier half of August there are found a number of 

 little sand-eels from 4 to 12 mm. in length, living apparently 

 near the bottom and evidently the progeny of the eggs deposited 

 in May or June. Those caught are far fewer than in the March 

 series, and it is probable that this is due to the eggs in the 

 June spawning-period being laid in water too shallow for 

 ordinary trawling operations and the swarms of larvae remaining 

 for some time in the same situation. 



The March swarms are caught in greatest numbers at f to 

 1 mile from shore and those in July in quite shallow water. 



These facts probably find their explanation in different 

 conditions of existence in the two periods of the year. In the 

 calm summer months the eggs can be laid without risk in 

 the sand of the shallows, where a high temperature favours 

 development, whereas in the winter months the eggs, laid by 

 the parent in the sand of the deeper water, avoid all risk from 

 storms or great reduction of temperature. It may therefore be 

 a simple case of the adaptation of an animal, guided by its 

 instincts, to its changed surroundings. We may notice the 

 same kind of phenomena in allied species, in which those 

 spawning earlier in the year have a spawning-place further out 

 to sea and those spawning late a spawning-place much nearer 

 inshore, e.g. cod, haddock, and whiting. 



We cannot trace these later swarms through the mid- water 

 to the surface and we are unable therefore to say with certainty 

 that they follow the same course as their predecessors of the spring. 

 A little reflection will show that the latter pass their sojourn 

 in the surface-water in the warmest summer months, whereas 

 the approach of winter might render it an unfit habitation for 

 the former. Under these circumstances it would not be sur- 

 prising to find that the late summer brood do not travel to the 

 offshore surface-water, but as it were, take a short cut in the 



