The Torah specifies that the Passover offering must come "from the lambs and from the goats" (Exodus 12:5). Does this mean both species are required together, or can either one suffice on its own?
The Mekhilta answers decisively: each species is valid independently. You may bring a lamb alone or a goat alone. The verse's use of "and" does not create a joint requirement.
The proof comes from (Leviticus 1:10), which discusses burnt-offerings: "And if from the sheep is his offering." This verse uses "if" and "from," indicating that a single species suffices for a burnt-offering. The Mekhilta then constructs an a fortiori argument — a kal va-chomer. If a burnt-offering, which carries higher sanctity, is valid when brought from only one kind of animal, then the Passover offering, which has lower sanctity, should certainly be valid from one kind alone.
Given this logical proof, the original question returns: if we can already derive the answer through reason, why did the Torah bother specifying "from the lambs and from the goats" at all? The answer is that the verse's purpose is to clarify that each species is independently valid — "of each in itself." Without this explicit statement, someone might have argued that the a fortiori reasoning was flawed or that the Passover had unique requirements. The Torah preempted the objection by stating the rule directly, leaving no room for doubt.