Erasmus Darwin (1731-1802). The reference is almost certainly to his last work, The Temple of Nature; or, the Origin of Society: A Poem, with Philosophical Notes, published posthumously in 1803.
Its cryptic citation in the opening paragraph of the Preface testifies to the major importance of this work for the conceptual structuring of Frankenstein, particularly in the electromagnetic linkage of the scientific concerns of Victor Frankenstein and Robert Walton, as well as for the conspicuous and strange polar setting of the novel. The relevant note is the twelfth in the appendix.
What exactly Percy Bysshe Shelley is referring to in his glancing citation of The Temple of Nature is harder to discern. That he knew the work intimately can be discerned by how much its form, as well as its science, contribute to the underlying conception of Queen Mab (1813) and its two-book redaction published in the Alastor volume in March 1816, "The Daemon of the World." In respect to Frankenstein, he is probably thinking of Darwin's notion of creation as occuring from the dynamic interaction of polar opposites in Book I.227ff, or its extension in the notion of life and death as interacting forces in Book IV.375ff. Likewise, of relevance (though wholly erroneous in its suppositions) is the first of the Additional Notes in the appendix, on "Spontaneous Vitality of Microscopic Animals." Darwin also has a curious exposition of male reproduction in nature without the intercession of females: see Book II, section III, somewhat elaborated in the eighth of the Additional Notes.