The world as a clock
Things in nature don't have parts; they are not the summation of subcomponents that can be replaced with other similar parts.
The parts we see in nature are just an artifact of the tool we use to understand nature; they are our comprehension of the world.
Where do leaves end and branches start? Where do branches end and the tree's trunk start? And what about the roots? And the bacteria that fixes the nitrogen from the soil into the roots? And the the soil? And the land? And the valley and the mountains that surround the valley?
The only things that are made out of exchangeable parts are the stuff we manufacture and that is precisely because they are crystallizations of our thoughts which are compounded by discrete parts.