290 A HISTORY OF 



some of the fens, till overcharged with numbers, they a ? periodical^ 

 obliged to migrate. An idea may be had of their nurnbers. when we 

 are informed, that a man, employed by a farmer to take them, for the 

 purpose of manuring his grounds, has got, for a considerable time 

 four shillings a day, by selling them at a half-penny a bushel ! 



Thus we see the amazing propagation of fishes along our own 

 coasts and rivers ; but their numbers bear no proportion to the vast 

 quantities found among the islands of the Indian ocean. The inha- 

 bitants of these countries are not under the necessity even of provid- 

 ing instruments for fishing ; it is but going down to the shore, and 

 there the fish are found in great numbers in the plashes that still con- 

 tinue to have water in them. In some of these places the quantity is 

 so great, that they are left in shoals, on those swamps, dried up by the 

 sun, and their putrefaction contributes to render the country un- 

 healthful. 



This power of increasing in these animals exceeds our idea, as it 

 would, in a very short time, outstrip all calculation. A single herring, 

 if suffered to multiply unmolested and undiminished for twenty years, 

 would show a progeny greater in bulk than ten such globes as that 

 we live upon. But happily the balance of Nature is exactly pre- 

 served ; and their consumption is equal to their fecundity. For this 

 reason we are to consider the porpoise, the shark, or the cod-fish, not 

 in the light of plunderers and rivals, but of benefactors to mankind 

 Without their assistance, the sea would soon become overcharged 

 with the burden of its own productions ; and that element, which at 

 present distributes health and plenty to the shore, would but load it 

 with putrefaction. 



In the propagation of all fish, some degree of warmth seems abso- 

 lutely necessary, not only to their preservation, but to the advance- 

 ment of their posterity. Their spawn is always deposited in those 

 places where the sunbeams may reach them, either at the bottom of 

 shallow shores or floating on the surface in deeper waters. A small 

 degree of heat answers all the purposes of incubation, and the animal 

 issues from the egg in its state of perfect formation, never to undergo 

 any succeeding change. 



Yet, still I have some doubts whether most fish come from the egg 

 completely formed. We know that in all the. frog tribe, and many of 

 the lizard kind, they are produced from the egg in an imperfect form. 

 The tadpole, or young frog, with its enormous head and slender tail, 

 are well known ; a species of the lizard also, which is excluded from 

 the shell without legs, only acquires them by degrees, and not till after 

 some time does it put off its serpent form. It is probable that some 

 kinds of fish in like manner suffer a change ; and though it be too 

 inconsiderable to strike the fisherman or the inattentive spectator, yet 

 it makes a very material difference to the naturalist, and would per- 

 haps disarrange his most favourite systems. A slight alteration in the 

 fins or bones that cover the gills would overturn the whole fabric of 

 the most applauded ichthyologist ; and yet, as I observed, it is most 

 probable that these minute alterations often take place. 



As a proof of this, during the month of July, there appear near 

 Greenwich innumerable shoals of small fishes, which are known to 

 the Londo lers by the name of White Bait. It is universally agreed 



