He can suppose that the womb of mother earth as it first emerged, like a huge animal, from its chaotic state, gave birth to creatures whose form displayed less purposiveness, and that these again bore others which adapted themselves more perfectly to their native surroundings and their relations to each other, until this womb, becoming rigid and ossified, restricted its birth to definite species incapable of further modification, and the multiplicity of forms was fixed as it stood when the operation of that fruitful formative power had ceased.