For if each part is liable to individual variations at all ages, and the variations tend to be inherited at a corresponding or earlier age--propositions which cannot be disputed--then the instincts and structure of the young could be slowly modified as surely as those of the adult; and both cases must stand or fall together with the whole theory of natural selection.

Some species of Molothrus, a widely distinct genus of American birds, allied to our starlings, have parasitic habits like those of the cuckoo; and the species present an interesting gradation in the perfection of their instincts.

The sexes of Molothrus badius are stated by an excellent observer, Mr.

Hudson, sometimes to live promiscuously together in flocks, and sometimes to pair.

They either build a nest of their own or seize on one belonging to some other bird, occasionally throwing out the nestlings of the stranger.