-disused habit I can scarcely doubt. I may mention
that twice I have seen birds of this species attempting to build
nests, and that on both occasions they failed to complete the work. So
universal is the nest-making instinct that one might safely say the
_M. bovariensis_ had once possessed it, and that in the cases I have
mentioned it was a recurrence, too weak to be efficient, to the
ancestral habit." Mr. Hudson suggests that this bird lost the
nest-making instinct by acquiring the semi-parasitical habit, common
to many South American birds, of breeding in the large covered nests
of the _Dendrocolaptidae_, although, owing to increased severity in the
struggle for the possession of such nests, this habit was
defeated.[81]

[81] P. L. Sclater and W. H. Hudson, _Argentine
Ornithology_, 1888, vol. i. pp. 72-86. A brief summary of
the facts regarding parasitism among birds will be found in
Girod's _Les Societes chez les Animaux_, 1891, pp.
287-294.

The _Rhodius anarus_, a fish of European rivers, also ensures a quiet
retreat for his offspring by a method which is not less indiscreet. At
the period of spawning, a male chooses a female companion and with
great vigilance keeps off all those who wish to approach her. When the
laying becomes imminent, the _Rhodius_, swimming up and down at the
bottom of the stream, at length discovers a _Unio_. The bivalve is
asleep with his shell ajar, not suspecting the plot which is being
formed against him. It is a question of nothing less than of
transforming him into furnished lodgings. The female fish bears
underneath her tail a prolongation of the oviduct; she introduces it
delicately between the Mollusc's valves and allows an egg to fall
between his branchial folds. In his turn the male approaches, shakes
himself over it, and fertilises it. Then the couple depart in search
of another _Unio_, to whom to confide another representative of the
race. The egg, well sheltered against dangers from without, undergoes
development, and one fine day the little fish emerges and frisks away
from his peaceful retreat.

Other animals, more respectful of property, avoid using another's
dwelling until it is abandoned by its proprietor, and no reproach of
indelicacy can be addressed to the _Gobius minutus_, a fish which
lives on our coasts at the mouth of rivers. The female lays beneath
overturned shells, remains of Oysters, or Cardium shells. The valve is
buried b