336 TROPICAL NATURE Iv 
little white-eyes (Zosterops), which are probably allied to the 
last, eat soft fruits and minute insects. 
Conclusion 
Here, then, we have an extensive group of birds, consider- 
ably varied in external form, yet undoubtedly closely allied 
to each other, one division of which is specially adapted to 
feed on the juices secreted by flowers and the minute insects 
that harbour in them; and these alone have a lengthened bill 
and double tubular tongue, just as in the humming-birds. 
We can hardly have a more striking example of the necessity 
of discriminating between adaptive and purely structural 
characters. The same adaptive character may coexist in two 
groups which have a similar mode of life, without indicating 
any affinity between them, because it may have been acquired 
by each independently to enable it to fill a similar place in 
nature. In such cases it is found to be an almost isolated 
character, apparently connecting two groups which otherwise 
differ radically. Non-adaptive or purely structural charac- 
ters, on the other hand, are such as have probably been 
transmitted from a remote ancestor, and thus indicate funda- 
mental peculiarities of growth and development. The changes 
of structure rendered necessary by modifications of the habits 
or instincts of the different species have been made to a great 
extent independently of such characters; and as several of 
these may always be found in the same animal their value 
becomes cumulative. We thus arrive at the seeming paradox 
that the less of direct use is apparent in any peculiarity of 
structure, the greater is its value in indicating true, though 
perhaps remote, affinities ; while any peculiarity of an organ 
which seems essential to its possessor’s wellbeing is often of 
very little value in indicating its affinity for other creatures. 
This somewhat technical discussion will, it is hoped, enable 
the general reader to understand some of the more important 
principles of the modern or natural classification of animals as 
distinguished from the artificial system which long prevailed. 
It will also afford him an easily remembered example of those 
principles, in the radical distinctness of two families of birds 
often confounded together,—the sun-birds of the Eastern 
‘Hemisphere and the humming-birds of America; and in 
