Page:Popular Science Monthly Volume 77.djvu/100

94 hitching its way, thus laden, up the wall of the nest, throws them overboard. If such a bird is replaced after a time, the same movements are repeated. With the coast thus clear, the little "parasite" can monopolize the attention of its nurse, and grows apace, being attended with all the care which is bestowed on a legitimate child. As Philemon Holland has quaintly rendered the account of the elder Pliny:

It is evident that this practise of nest-stealing, somewhat ambiguously called "parasitism," could never become very popular or widespread, for it would soon break down of its own weight.

For over two thousand years, or since the time of Aristotle, who was the first to leave a permanent record of this propensity in the European cuckoo, the question has been asked, How could such a habit arise? and the answers have been various, and far from satisfactory. The key to the matter lies, as we believe, in the cyclical instincts, and in the disturbances to which they are prone. When the normal rhythm is generally disturbed or permanently changed, new instincts and even new structures may arise, which serve as a counterbalance to the changes wrought.

We believe that the instinct of parasitism got its start through lack of attunement in terms 3 and 4, of the reproductive cycle, and that it has passed through essentially the following stages: (1) The egg forthcoming before there is a nest ready to receive it, a condition sporadic in very many, if not in most modern birds, due to unknown causes, such as lead to a premature growth of the ovary, or to a disturbance of certain instincts. There is a loss of eggs, although a nest may be eventually built, and young reared in the season. (2) The eggs are ready before the nest, and many are lost by dropping them on the ground, while others are laid in stolen nests. A proper nest is sometimes built, but whether young are ever reared, will depend upon circumstances. This stage is exemplified by the Argentine cowbird (Molothus badius), described by Hudson, which commonly wastes its eggs, scattering them in all directions, yet it will steal a nest upon