OF SELBOKNE 83 



The grasshopper-lark chirps all night in the height of summer. 1 



Swans turn white the second year, and breed the third. 



Weasels prey on moles, as appears by their being sometimes 

 caught in mole-traps. 



Sparrow-hawks sometimes breed in old crows' nests, and the 

 kestril in churches and ruins. 



There are supposed to be two sorts of eels in the island of Ely. 

 The threads sometimes discovered in eels are perhaps their 

 young : the generation of eels is very dark and mysterious. 2 



Hen-harriers breed on the ground, and seem never to settle on 

 trees. 



When redstarts shake their tails they move them horizontally, 

 as dogs do when they fawn : the tail of a wagtail, when in motion, 

 bobs up and down like that of a jaded horse. 3 



Hedge-sparrows have a remarkable flirt with their wings in 

 breeding-time ; as soon as frosty mornings come they make a 

 very piping plaintive noise. 



Many birds which become silent about Midsummer reassume 

 their notes again in September ; as the thrush, blackbird, 4 wood- 



1 [" Chirp" is an inappropriate word for the reel of the grasshopper-warbler. 

 White was probably at a loss for a verb to express the peculiar stridulation which 

 he had well described in Letter XVI. to Pennant.] 



2 [The most important new light upon the generation of eels is that furnished 

 by the observations of Grassi and Calandruccio (Atti della Reale Accademia dei 

 Lincei, vol. vi., 1897). It was long ago ascertained that the eel descends to the 

 sea in autumn or early winter, and that it never spawns, nor even becomes mature 

 in fresh waters. The eels which descend to the sea never return, but young eels or 

 elvers come up from the sea in spring, millions at a time. The elvers have been 

 seen to travel along the bank of a river in a continuous band or eel-rope, which has 

 been known to glide upwards for fifteen days together. We now know something 

 of that part of the history which is transacted in the sea. When it leaves the river 

 the eel makes its way to very deep water, and there undergoes a change. The eyes 

 dilate ; the pectoral fin and the border of the gill-cover turn black ; the reproductive 

 organs enlarge. The eels lay their eggs in water of not less than 250 fathoms' 

 depth, the young which are hatched out being tape-like, transparent, colourless, 

 devoid of red blood, and armed with peculiar teeth. Such fishes had been captured 

 from time to time, and described as a peculiar kind of deep-water fish, to which 

 the name of Leptocephalus was given. After a time the Leptocephalus ceases to feed, 

 loses bulk, and develops pigment on the surface of the body. The larval teeth are 

 cast, and the larval skeleton is replaced by a new one. Then the fish comes to the 

 surface, enters the mouth of a river, and if caught is immediately recognised as an 

 elver. It is now a year old, and about two inches long.] 



3 [The " flicker " of the redstart's tail has been a curious puzzle to ornithologists. 

 Macgillivray, most careful of observers, affirmed that it was a vertical, not a hori- 

 zontal movement, and Seebohm follows him. Mr. Harting and others agree with 

 White. The fact is that the motion is so quick that it is almost impossible to be 

 quite sure of the way in which it is done.] 



4 [This is probably a slip, and the missel-thrush may have been meant ; the 

 blackbird rarely, if ever, sings after the moult. The question of the cause of 

 autumn song, which White here raises, has never yet been scientifically explained.] 



