DWELLINGS. 1 45 



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 

 Dendrocolaptidce, although, owing to increased severity 

 in the struggle for the possession of such nests, this 

 habit was defeated.^ 



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 spawn- 

 ing, 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 Unto. 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 under- 

 neath 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 Unto, to' whom to confide another repre- 

 sentative of the race. The egg, well sheltered against 



1 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 Sociith chez les Animaux, 



1 891, pp. 287-294. 



10 



