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Show 1874.] MOLOTHRI OF BUENOS AYRES. 167 from it when found, is a contingency for which no provision has been made; consequently the egg is wasted. In the Rheas all the females in a flock lay in one nest, a male incubating the eggs afterwards. But as each bird lays a large number of eggs, and as they do not begin laying at the same time, long before they have all done laying the male becomes broody and drives them away. This is probably the cause of their wasting so many eggs ; for all the females that are ready to lay when the male begins to incubate are compelled to drop them upon the plain. But how can we account for the habit of occasionally wasting eggs in another species-the Urraca (Cyanocorax pileatus) for example ; for this bird builds an elaborate nest in which but one female lays ? In a paper on the habits of this species (P. Z. S. 1870, p. 749) I said that they sometimes built such frail nests that all the eggs dropped through them ; but I did not then know that they also wasted a surprising number of eggs. A flock of about sixteen of these birds passed thewinter of 1872 in thetrees about my house ; on the approach of warm weather they began to scatter, incessantly screaming and chattering as their manner is when about to pair and breed. I observed these birds very attentively, but could not detect them building. At last I found three broken eggs on the ground, and on examining the tree overhead discovered an incipient nest, merely a dozen or so of little sticks laid crossways, upon, or rather through, which the eggs had been dropped. This was in October; and till January no other attempt at a nest was made ; but eggs in abundance were wasted, for during four months I constantly found them about the orchard. Early in January another nest was made, but with less materials than a Cuckoo would have employed ; five or six smashed eggs were on the ground beneath it. Towards the end of January two large nests were built, deep and well-lined with green leaves ; in these nests fourteen or fifteen birds were hatched. In this case we see one essential link in the chain of procreant instincts struck out and the whole made abortive; but the cause of the loss or suspension for a time of the architectural habit seems very mysterious. Besides the Rhea, I know of no species in which two or several females unite habitually to lay in one nest; but there are many species in which two or more females occasionally lay together. This is often the case with Dendrocygnafulva, Vanellus cayennensis, and Nothura maculosa. I can give but one case of two birds of different species laying together: this was the Teal (Querquedula flavirostris) and the common Partridge or Tinamou (Nothura maculosa). The nest was in a thistle-bush at a distance from the water, and contained the full complement of eggs of both birds. XII. Conjectures as to the origin of the parasitic instinct in M . bonariensis.- The assertion that the " immediate and final cause of the Cuckoo's instinct is that she lays her eggs not daily, but at intervals of two or three days" (' Origin of Species'), carries no great appearance of probability with it; for might it not just as reasonably be said that the parasitic instinct is the immediate and final cause of her laying her eggs at long intervals ? If it is favourable to a species with the |