[jongjong]

It's wild to think that plants are engaged in this constant struggle to produce seeds that have an outer shell that is just strong enough not to be consistently dissolved in a bird's stomach but not so strong that they won't ever dissolve.

One one hand, some seeds must survive passing through the bird's digestive system intact to later grow into a plant, on the other hand, some seeds must be digested in order to keep the birds interested in consuming that seed... Alternatively, a bird species interested in eating indigestible seeds may become extinct due to malnutrition.