No system can possibly be formed, even in imagination, without a subordination of parts.
Even animal body must have different members subserviant to each other; every picture must be composed of various colours, and of light and shade...
It would have been no more an instance of God's wisdom to have created no beings but of the highest and most perfect order, than it would be of a painter's art to cover his whole piece with one single colour, the most beautiful he could compose.
Had he confined himself to such, nothing could have existed but demi-gods, or arch-angels, and then all inferior orders must have been void and uninhabitated: but as it is surely more agreeable to infinite Beneveloence, that all these should be filled up with beings capable of enjoying happiness themselves, and contributing to that of others, they must necessarily be filled with inferior beings, that is, with such as are less perfect, but from whose existance, notwithstanding that less perfection, more felicity upon the whole accrues to the universe, than if no such had been created.
It is moreover highly probable, that there is such connection between all ranks and orders by subordinate degrees, that they mutually support each other's existance, and every one in its place is absolutely necessary towards sustaining the whole vast and magnificent fabrick.

(Soame Jenyns, quoted by Samuel Johnson, Review of a free Enquiry, 1757.)