We can read up on species with Roger Bacon here:
We find there:
There is also an ontological difference between virtus and species: virtus is a real being, and the capacity for the full realization of a potency; a species is its first effect, having a deficient being. Virtus exists absolutely in the medium; species exist there intentionally. Bacon had a unique understanding of the meaning of the ‘intentional’ existence of the species in medio. While among his contemporaries ‘intentional’ was considered equivalent to ‘spiritual’ (though not necessarily ‘mental’) and opposed to ‘natural’, Bacon thought of ‘intentional’ as having a weak and incomplete being.54 A species, Bacon wrote, in relation to a ‘real’ being is so deficient that it cannot be enumerated among the things of this world. It “is not called a thing, but more the similitude of things”.55
and
A species does not advance in the medium by locomotion; it regenerates itself in consecutive parts of the medium. The production of a species, Bacon explains, involves a true and natural transmutation of the substance of the patient, which is made by true generation (per veram generacionem). The patient in this case is any receiver of a species, be it the medium or the final recipient.64 A virtus, by contrast, passes through all sorts of mediums without affecting them; it affects only the substances predisposed to receive its influence.
Very much simplified: Bacon's 'species' are not things. But certain things have the 'virtus' to produce 'species' in a kind of Aristotelean "emanation", which pass through an adequate medium and affect a predisposed substance.
That works well to describe sensory species and was likely their inspiration. But also Bacon's species are not particles.