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IOWA ACADEMY OP SCIENCE 
creature that remains in their path, can not fail at times to come upon nests 
that are placed on or near the ground. These ants do not usually ascend far 
into the trees nor go out to the ends of long branches; it may be partly for this 
reason that some of the Manikins and other small birds nesting near the ground 
place their nests on long, slender twigs. 
To some of the foregoing examples as illustrative of protective adaptation, 
it may be objected that individual cases occur where the very element is want- 
ing which renders the peculiar structure or location of the nest protective. 
For instance, the nest of Onychorhynchus does not always overhang a stream, 
and may even be placed far above the level of the highest flood; the nest of 
RhyncTiocyclus is not always in a thorny Acacia; Myiozetetes and Tanagra cana 
sometimes build their nests far from that of Pitangus, etc. It can only be an- 
swered that -in analagous cases of adaptation throughout nature we will And 
the same sort of exceptions; and that the positive evidence is so largely in pre- 
ponderance of the negative as to be obvious to any ordinary observer. 
Anyone who has given the slightest attention to the breeding habits of birds 
is familiar with the fact that there is a wide range of individual variation 
within the limits of almost any species; and it is no less true that in cases 
where highly specialized nonstructural adaptations of any kind occur, the range 
of individual variation is likely to be still wider. We can not, in any of the 
foregoing cases, regard the protective adaptations as dependent on perfectly 
rigid and deflnite laws of action, as in the case for instance with the migration 
of birds. Natural selection is still, doubtless, pre-eminently operative in com- 
pelling conformity to a set of peculiar conditions, whose very complicity implies 
immense variations in the effort, conscious or unconscious, to meet them. 
Whether these variations are dependent on slight structual differences, age, mere 
accident, or some other circumstance or combination of circumstances, is a 
matter very difficult to determine. 
