MOLOTHRUS BADIUS. 95 



should we attempt to mentally create a perfect parasitical instinct 

 (that is, one that would be thoroughly efficient with the least 

 possible prejudice to or injustice towards another species ; for the 

 preservation of the species on which the parasite is dependent is 

 necessary to its own) by combining in imagination all known parasitical 

 habits, eliminating every offensive quality or circumstance, and attri- 

 buting such others in their place as we should think fit, our conception 

 would probably still fall short in simplicity, beauty, and complete- 

 ness of the actual instinct of M. rufoaxillaris. Instead of laying its 

 eggs promiscuously in every receptacle that offers, it selects the nest 

 of a single species ; so that its selective instinct is related to the 

 adaptive resemblance in its eggs and young to those of the species on 

 which it is parasitical. Such an adaptive resemblance could not of 

 course exist if it laid its eggs in the nests of more than one species, 

 and it is certainly a circumstance eminently favourable to preservation. 

 Then, there not being any such incongruity and unfitness as we find in 

 nests into which other parasites intrude, there is no reason here to 

 regard the foster-parents' affection as blind and stupid; the similarity 

 being close enough to baffle the keenest sagacity. Nor can the instinct 

 here appear in the light of an outrage on the maternal affection ; for the 

 young M. rufoaxillaris possesses no advantage over its foster-brothers. 

 It is not endowed with greater strength and voracity to monopolize the 

 attentions of the foster-parent or to eject the real offspring ; but being 

 in every particular precisely like them, it has only an equal chance of 

 being preserved. To this wonderful parasitical instinct we may well 

 apply Darwin's words, when speaking of the architecture of the hive- 

 bee : " Beyond this stage of perfection natural selection could not 

 lead." 



96. MOLOTHRUS BADIUS, Vieill. 

 (BAY-WINGED COW-BIRD.) 



[PLATE VI. FIG. 1.] 



Molothrus badius, Burm. La-Plata Reise, ii. p. 495 (Parana and Tucuman). 

 Scl. et Salv. Nomencl. p. 37 ; Hudson, P. Z. S. 1874, p. 163 (Buenos Ayres) ; 

 Durnford, Ibis, 1877, p. 174 (Buenos Ayres) ; Scl. Cat. B. xi. p. 338. 



Description. Dull grey, beneath rather paler; wings chestnut: tips of 

 primaries, inner portions of secondaries, and tail blackish ; bill and feet black : 

 total length 7'6 inches, wing 3-5, tail 3-0. Female similar. 



Hab. Argentina, Paraguay, and Bolivia. 



In this species the sexes are alike ; the plumage of the body is grey- 



