by Merry — Early in child­hood, we learned that the plants and an­i­mals around us are dis­crete en­ti­ties with def­i­nite bound­aries. A dog might have fleas, but fleas and dogs are sep­a­rate and dis­tinct in­di­vid­u­als. With our in­creased fa­mil­iar­ity with sym­bioses – es­pe­cially those of the ob­lig­ate sort – have come many in­stances where...