I have a problem with understanding of these sentences:

>We have indicated in the opening paragraph of the Introduction that surface plasmon polaritons are solutions
of Maxwell’s equations in which the effects of retardation—the finiteness of the speed of light—are
taken into account. An important subclass of surface plasmon polaritons are surface plasmons. These can
be viewed as the limiting case of surface plasmon polaritons when the speed of light is allowed to become
infinitely large.
>
>Nano-optics of surface plasmon polaritons (1.1.2) - Anatoly Zayats et.al.

How can we allow c be infinite? Why is it correct in case of surface plasmons?