Refer to this line in the passage "According to Aristotle, the former have their principles of generation and motion inside, whereas the latter, insofar as they are artifacts, are generated only by outward causes."
Here, 'former' refers to 'natural things' while 'latter' refers to 'artifacts'.
Aristotle gives the example of animals to prove his point- they are capable of growth, movement, and reproduction.
So, natural things have their principles of generation and motion present inside them.