Complex sentence: LD figures depend not only on the species of animal, but also on the mode of administration of the toxin; for instance, a toxin inhaled or injected into the bloodstream may require a much smaller dosage than if the same toxin is swallowed.
Split into simpler sentences: LD figures depend not only on the species of animal, but also on the mode of administration. For instance, a toxic substance inhaled or injected into the bloodstream may require a much smaller dosage than if the same substance is swallowed.