Bardo of Becoming
If we miss the opportunity this life offers us for transforming ourselves, they say, it may well be an extremely long time before we have another. Imagine a blind turtle, roaming the depths of an ocean the size of the universe. Up above floats a wooden ring, tossed to and fro on the waves. Every hundred years the turtle comes, once, to the surface. To be born a human being is said by Buddhists to be more difficult than for that turtle to surface accidentally with its head poking through the wooden ring.
Rinpoche, Sogyal (2009-10-13). The Tibetan Book of Living and Dying: The Spiritual Classic & International Bestseller: Revised and Updated Edition (Kindle Locations 2310-2313). Harper Collins, Inc.. Kindle Edition.