The World in Morphic is the rectangular area which fills the whole screen.
Technically it is a global variable which keeps the instance of the topmost PasteUpMorph in the The Future of Squeak, 1999. All other morphs are submorphs (or submorphs of submorphs) of this Morph.
Any Morph answers the message 'world' with this topmost instance of PasteUpMorph.
So all Morphs you see on a screen belong to one big composition hierarchy with the global variable World pointing to the topmost Morph, which is a PasteUpMorph.
See 'The Design Patterns Smalltalk Companion' by Alpert, Brown, Woolf, p. 137: Composite pattern.
Note: Morph corresponds to 'Component' there. But Leaf and Composite are not factored out. So Morph has all the functions of Component, Leaf and Composite.
To get an array with the components of a Morph you send it the message 'submorphs'.