"(5) … (b) in the case of a claim involving a new party, if the addition or substitution of the new party is necessary for the determination of the original action. (6) The addition or substitution of a new party shall not be regarded for the purposes of subsection 5(b) above as necessary for the determination of the original action unless either -- […] (b) any claim made in the original action cannot be maintained by or against an existing party unless the new party is joined or substituted as plaintiff or defendant in that action."
“The text is not quite the same as that of the section, but the rule must be construed as being no wider than is permitted by the section. The effect, on the facts of this case, is that paragraph (3)(b) has to be satisfied, it being shown 'that the claim cannot properly be carried on by or against the original party' without the substitution.”