144 The Naturalist in Nicaragua 
of the same genus, as in many insects, differing but little from 
each other, yet quite distinct, and we ask why, if these have 
descended from one parent form, do not the innumerable 
gradations that must have connected them exist also? 
There is but one answer; we are ignorant what characters are 
of essential value to each species; we do not know why white 
terriers are more subject than darker-coloured ones to the 
attacks of the fatal distemper; why yellow-fleshed peaches in 
America suffer more from diseases than the white-fleshed 
varieties ; why white chickens are most liable to the gapes ; or 
why the caterpillars of silkworns, which produce white 
cocoons, are not attacked by fungus so much as those that 
produce yellow cocoons? Yet in all these cases, and many 
others, it has been shown that immunity from disease is 
correlated with some slight difference in colour or structure, 
but as to the cause of that immunity we are entirely ignorant. 
At last we reached the summit of the range, which is prob- 
ably not less than three thousand feet above the sea, and 
entered on the district of Libertad. Rounded boggy hills 
covered with grass, sedgy plants, and stunted trees replaced 
the dry gravelly soil of the Juigalpa district. The low trees 
bore innumerable epiphytal plants on their trunks and 
boughs. Many of these are species of T7zllandsia, which sit 
perched up on the small branches like birds. They have 
sheathing leaves that hold at their base a supply of water 
that must be very useful to them in the dry season. Insects 
get drowned in this water, and the plants may derive some 
nourishment from their decomposing bodies, but I believe the 
principal object is to obtain a supply of moisture, as the roots 
of the plants do not hang down to the ground, like those of 
many other epiphytes in the tropics, nor are they provided 
with bulbs like the orchids. Some plants that hold liquids 
in cup-shaped leaves are simply insect traps, many of them 
growing in bogs, where the supply of moisture is perennial 
and constant. Such is the Indian-cup (Sarracenia) that 
erows in the bogs of Canada, and the Californian pitcher- 
plant (Darlingtonia californica), which also grows in bogs, 
and is such an excellent fly-trap, that there is generally a 
layer of from two to five inches of decomposing insects lying 
at the bottom of the cup. The different species of Drosera, 
1See Nature, vol. iii. pp. 159 and 167. 
