OF THE ANIMAL FRAME 121 



beyond the germinating influence of the warmth and air contained in the super- 

 natant water, communicated to it by the sun and the atmosphere. But the inde- 

 structible texture of the integument which enclosed the fecundated ova has 

 preserved them, perhaps for years, from injury and corruption ; and they have 

 only waited for that very exposure to light, air, and warmth, which the re- 

 moval of the superior stratum of mud has produced, to awaken from their 

 dormant state into life, form, and enjoyment ; and but for which they would 

 have remained in the same state, dormant but not destroyed, for ten or twelve 

 times as long a period. 



So, in the hollows upon our waste lands, when they have been for some 

 time filled with stagnant water, we not unfrequently find eels, minnows, and 

 other small species of the carp genus, leeches,* and water insects, and won- 

 der how they could get into such a situation. But the mud which has been 

 emptied out of the preceding fish-pond has perhaps been thrown into these 

 very hollows ; or the ova of the animals have been carried into the same 

 place by some more recondite cause ; and they have been waiting, year after 

 year, for the accidental circumstance which has at length arrived, and given 

 them the full influence of warmth, water, light, and air. 



The ova of many kinds are peculiarly light, and almost invisibly minute. 

 They are hence, when the mud, which has been removed from fish-ponds 

 becomes dry and decomposed into powder, swept by the breeze into the 

 atmosphere, from which they have occasionally descended into the large 

 tanks which] are made in India as reservoirs for rain-water; and producing 

 their respective kinds in this situation, have appeared, to the astonishment of 

 all beholders, to have fallen from the clouds with the rain itself. Dr. Thom- 

 son, in adverting to this curious fact, observes that it is difficult to account 

 for it satisfactorily.! The explanation now offered will, if I mistake not, 

 sufficiently meet the case. 



Many insects can only be hatched in a particular animal organ ; and it is 

 the office of the integument of the ovum to preserve it in a perfect state till 

 it has an opportunity of reaching its proper nidus. Thus the horse-gadfly, or 

 oestrus equi, deposites its eggs on the hairs of this animal, and sticks them to 

 the hair-roots by a viscous matter which it secretes for this purpose. But 

 here they could never be hatched, though they were to remain through the 

 whole life of the horse : their proper nidus is the horse's stomach or intes- 

 tines, and to this nidus they must be conveyed by some means or other ; and 

 in their first situation they must remain and be preserved, free from injury 

 or corruption, till they can obtain such a conveyance. The integument in 

 which they are wrapped up gives them the protection they stand in need of; 

 and the itching which they excite in the horse's skin compels him to lick the 

 itching part with his tongue ; and by this simple contrivance the ova of the 

 gadfly are at once conveyed to his mouth, and pass with the food into the 

 very nidus which is designed for them. 



It is the same integument that, by its incorruptibility, preserves the cater- 

 pillar during the torpitude of its chrysalid state, while suspended by a single 

 thread from the eaves of an incumbent roof; and which thus enables the 

 worm to be transformed into a butterfly. The larve of the gnat, when ap- 

 proaching the same defenceless state, dives boldly into the water, and is 

 protected by the same indestructible sheath from the dangers of an untried 

 element. 



In several species the insect remains in its chrysalid state for many years .- 

 the locust, in one of its species at least, the cicada septendecim, appears in 

 numbers once only in seventeen years, and the palmer-worm once only in 

 thirty years ; cycles not recognised by the meteorologist, but which are well 

 entitled to his attention : and, through the whole range of their duration, it 

 is the integument we are now speaking of that furnishes the animal with a 

 secure protection. 



Whence comes it that plants of distant and opposite climates (for every 



* See Willd. p. 120, note. t Annals of Philos. viii. p. 70. 



