The Physical World..

Know that the order and the perfection of the whole universe require that existence should appear in numberless forms. For existing beings could not be embodied in only one degree, one station, one kind, one species and one class; undoubtedly, the difference of degrees and distinction of forms, and the variety of genus and species, are necessary- that is to say, the degree of mineral, vegetable, animal substances, and of man are inevitable; for the world could not be arranged, adorned, organized and perfected with man alone. In the same way, with only animals, only plants or only minerals, this world could not show forth beautiful scenery, exact organization and exquisite adornment. Without doubt it is because of the varieties of degrees, stations, species and classes that existence becomes resplendent with utmost perfection...

Abdu'l-Baha, Some Answered Questions129