88 BIRDS OF LA PLATA 
on the subject, and will go so far as to suggest what 
may have been at least one of the many concurrent 
causes that have produced the parasitic instinct. 
The apparently transitional nesting-habits of several 
species, and one remarkable habit of M. bonariensis, 
seem to me to throw some light on a point bearing 
intimately on the subject, viz., the loss of the nest- 
making instinct in this species. 
Habits vary greatly; were it not so they would 
never seem so well adapted to the conditions of life 
as we find them, since the conditions themselves are 
not unchangeable. Thus it happens that, while a 
species seems well adapted to its state in its habits, 
it frequently seems not so well adapted in its rela- 
tively immutable structure. For example, without 
going away from the pampas, we find a Tringa with 
the habits of an upland Plover, a Tyrant-bird -(Pi- 
tangus bellicosus) preying on mice and snakes, another 
Tyrant-bird (Myiotheretes rufiventris) Plover-like in 
its habits, and finally a Woodpecker (Colaptes cam- 
pestris) that seeks its food on the ground like a 
Starling; yet in none of these—and the list might 
be greatly lengthened—has there been anything like 
a modification of structure to keep pace with the 
altered manner of life. But however much the 
original or generic habits of a species may have 
become altered—the habits of a species being widely 
different from those of its congeners, also a want of 
correspondence between structure and habits (the © 
last being always more suited to conditions than the 
first) being taken as evidence of such alteration— 
