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 (Paraná 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-drab colour, with a black spot between the eye and beak; tail dark, the quills cinnamon-colour; beak and legs black. Azara describing it under the name of _Tordo pardo roxiso_, says it is a rare bird, so that it has probably greatly increased since his time, as it is now quite common in the Plata district.