Each egg being so large as entirely to fill up the abdominal cavity and with difficulty pass the walls of the pelvis, a considerable interval is required before the successive eggs can be matured (the natives say about thirteen days).

Each bird lays six or eight eggs or even more each season, so that between the first and last there may be an interval of two or three months.

Now, if these eggs were hatched in the ordinary way, either the parents must keep sitting continually for this long period, or if they only began to sit after the last egg was deposited, the first would be exposed to injury by the climate, or to destruction by the large lizards, snakes, or other animals which abound in the district; because such large birds must roam about a good deal in search of food.

Here then we seem to have a case in which the habits of a bird may be directly traced to its exceptional organization; for it will hardly be maintained that this abnormal structure and peculiar food were given to the Megapodidae in order that they might not exhibit that parental affection, or possess those domestic instincts so general in the Class of birds, and which so much excite our admiration.
It has generally been the custom of writers on Natural History to take the habits and instincts of animals as fixed points, and to consider their structure and organization, as specially adapted, to be in accordance with these.

This assumption is however an arbitrary one, and has the bad effect of stifling inquiry into the nature and causes of "instincts and habits," treating them as directly due to a "first cause," and therefore, incomprehensible to us.