ANTS AND PLANT-STRUCTURE 407 
[When I compare these and similar instances 
with the Pitchers of the Nepenthes, in which (as I 
learn from the accounts of travellers) ants as well as 
water are nearly always found, I cannot doubt that 
those curious appendages have attained their actual 
dimensions through the deepening 'and widening 
which they have undergone from ants through 
untold ages.] 
We have a curious example, in the genus 
Cinchona, of the supposed correlation of a minute 
structural peculiarity with chemical and medical 
properties. Eminent botanists, such as Weddell 
and Karsten, who have studied that genus in its 
native forests, have thought they had found a char- 
acter in the leaves always associated with a bark 
rich in alkaloids, viz. the presence of a small pit or 
scrobicule in the axil of each vein on the underside 
of the leaf But when good specimens of C. silc- 
cirubra, the richest of all the barks in alkaloids, 
came to be examined, the leaves were found entirely 
destitute of scrobicules ! See now how this comes 
about. The leaves of the Hill Barks — those, namely, 
that grow at an elevation of 8000 feet and upwards 
— are liable to be infested by a small mite which 
nestles in the scrobicules — has caused them, in fact — 
its remote ancestors having at first sheltered in the 
vein-axils ; but C. succirtibi^a grows always below 
that elevation — indeed, as low down as 2400 to 6000 
feet — and is the only quinine-producing Cinchona 
that descends so low, the other species of Cinchona 
that grow at a low elevation having all medically 
worthless bark. But as all these species, C. sttc- 
cirttbra included, are equally destitute of scrobiculate 
leaves and of mites, the reasonable inference is that 
