388 NATURAL SCIENCE. June. 



in temperature of the surrounding medium.^ Thus, especially when 

 dealing with the lower temperatures, many days or weeks may be 

 added to the length of the period of incubation by lowering the 

 temperature a few degrees. A determination, by direct experiment, 

 of the period of incubation of a given species at various temperatures, 

 and a series of determinations of the mean surface-temperature in 

 any given district for different periods of the year, would thus make it 

 possible to state the incubatory-period for the species in that district. 

 Thus if the spawning-period be late, development will be rapid, and 

 the pelagic-period will be short. It follows from this that the time 

 of drift must be reduced, and, the drift-current being beyond control, 

 the spawning-area (A) must be further inshore. In the case of bulk, 

 it is found as a general rule that the greater the size of the egg the 

 longer is the period of incubation ; so that in relation to this factor, 

 the spawning-area (A) will be fixed in position according to the 

 specific size of the egg. Each species will have its spawning-area 

 determined by the size of the egg. 



Lastly, we have the factor of specific gravity .^ Pelagic eggs 

 invariably sink in fresh, and usually do so in brackish water. Hence 

 a fresh-water fish with eggs and larvae suited to a pelagic existence 

 will have to be katadromous in habit, that is, it must have a 

 spawning migration seawards. The eel and, to a less extent, the 

 flounder may be cases in point. Again, there are relationships of the 

 eggs to their medium of which we know very little. The pelagic 

 faunas of Crustacea and Mollusca are known to be susceptible to 

 climatic conditions, and at different times to be found at the surface, 

 in mid-water, or even deeper. Though the modus operandi is not so 

 intelligible, there are facts indicating that, under certain conditions 

 of which we know nothing, the masses of pelagic eggs of fishes are to 

 be found now at the surface, at other times in the mid-water and, 

 again, at the bottom. In each case the}'- are perfectly healthy and 

 developing normally. All that can be said with certainty is that 

 under some conditions the specific gravity of the eggs relative to that 

 of the sea-water is altered, resulting in a sinking or rising. In the 

 Baltic, the eggs of the plaice have been found far below the surface, 

 floating underneath the stratum of brackish water. 



The various factors of wind, temperature, and specific gravity 

 have thus been briefly touched upon, and their relationship to the 

 important economic question of the determination of spawning-areas 

 has been shewn. Thus, for the most successful results in the 

 survival of the brood, the spot A in a given area muse be selected by 

 the particular species and must be determined by these factors, 

 amongst others : — 



1 H. Dannevig. Rep. Fisk. Board Scotland, xiii., p. 147; 1895. 



2 C/. J. Hjort. " The distribution of the eggs of pelagic fishes therefore follows 

 the same laws which govern all the Plankton, it is dependent on the amount of 

 saltness and the currents." Studies of the Norwegian Fisheries, 1896. 



