Dogen says, quoting Buddha,
If you want to understand the true meaning of the buddha-nature, you should correctly understand its momentary manifestations.
You are all its momentary manifestation. Everything in the world is its momentary manifestation. Somewhere nature has blossomed into a rose, somewhere it has become a bird flying in the sky, somewhere it is a pine tree reaching to the stars, and somewhere it is a human being. These are all momentary manifestations of the same nature.
The word ‘buddha’ comes from the Sanskrit root buddh. Buddh means awareness. In any form you can become aware. But the human form is the easiest one from which to become aware. If you miss this opportunity you are missing something that you may find only after millions of years of search. Being a pine tree or a mountain rock – these are all manifestations. But no mountain has become a buddha, and no pine tree in its tremendous beauty has ever become enlightened. No animal, no bird, no tree, no sun, no moon, in all their beauty… they are manifestations of the same nature, but only man is capable of becoming aware of this self-nature. This double awareness – awareness of awareness – is man’s grandeur. It is his treasure.

Dogen, the Zen Master: A Search and a Fulfillment, Ch 6, Ch 3 (excerpt)