But at the same time a universal will be instantiated in more than one place, and this would seem to deprive it of its unity. Everything is particular just by its own nature. But for this view to work, the nature of an individual would have to be intrinsically individual. And Scotus denies this. There are 6 or 7 arguments, depending on how you count the last one.
Every genus requires some one thing that is the measure of everything in that genus.
This principle is introduced as Aristotle's, but it's independently plausible. But think of frogs being decimated by pollution. This measure must have real unity, because otherwise it couldn't be a measure. So there must be some real commonality at the species level. Sc. is limiting his argument not just to species and genera, but to species alone.