76 NATURAL HISTORY 
summer see numbers crawling out of the pools 
where they are hatched up the dry banks. There 
are varieties of them, differing in colour; and some 
have fins up their tail and back, and some have 
not. 
LETTER XIX. 
Selborne August 17, 1768. 
Dear Sir,—I HAVE now, past dispute, made out 
three distinct species of the willow-wrens (motaci/- 
le trochili), which constantly and invariably use dis- 
tinct notes. But, at the same time, I am obliged to 
confess that I know nothing of your willow-lark.* 
In my letter of April the 18th, I had told you per- 
emptorily that I knew your willow-lark, but had not 
seen it then; but, when I came to procure it, it 
proved inall respects a very motacilla trochilus, only 
that it is a size larger than the two other, and the 
yellow-green of the whole upper part of the body 
is more vivid, and the belly of a clearer white. [ 
have specimens of the three sorts now lying before 
me, and can discern that there are three grada- 
tions of sizes, and that the least has black legs, 
and the other two flesh-coloured ones. ‘The yel- 
lowest bird is considerably the largest, and has its 
quill-feathers and secondary feathers tipped with 
white, which the others have not. ‘This last haunts 
only the tops of trees in high beechen woods, and 
makes a sibilous, grasshopper-like noise now and 
then, at short intervals, shivering a little with its 
* Brit. Zool., edit. 1776, 8vo, p. 381. : 
