But in all cases, speculation on an instinct known to us only in a single species, is useless, for we have hitherto had no facts to guide us.

Until recently the instincts of the European and of the non-parasitic American cuckoo alone were known; now, owing to Mr.Ramsay's observations, we have learned something about three Australian species, which lay their eggs in other birds' nests.

The chief points to be referred to are three: first, that the common cuckoo, with rare exceptions, lays only one egg in a nest, so that the large and voracious young bird receives ample food.

Secondly, that the eggs are remarkably small, not exceeding those of the skylark--a bird about one-fourth as large as the cuckoo.

That the small size of the egg is a real case of adaptation we may infer from the fact of the mon-parasitic American cuckoo laying full-sized eggs.