Equity Suits On Behalf Of Numerous Persons

Today I ran across this passage in an older equity treatise. 1 Robert Treat Whitehouse, Equity Practice: State and Federal 92-96 (1915) (emphases added):

§ 59. Numerous persons. The cases where of numerous persons having a material interest, a portion may be dispensed with as parties, rest on the principle of virtual representation, i. e., the principle that where a large number of persons have a common interest, a portion of the number bringing a bill or defending in behalf of themselves or others may fairly be taken to represent the whole so that a decree can be rendered in the case without prejudice to the rights of the absent. The interest in question must be a community of interest in the subject matter of the suit, i. e., in