They fail to account for the fixed nature of animals.

We turn, again, to examine that thing which they also investigate, (namely), of what use are harmful creeping things which have been created. But being eager to win, they have been quickly defeated. For how does a creeping thing do harm, seeing that even in it, as they have said, there is mixed in it some of the Good Nature which is scattered through everything? And where is the Evil that is not mixed in an innocent lamb, if it is scattered in everything? And so it is possible to distinguish between Good and Evil by means of wolves and lambs, and by means of serpents and doves, and the Mixing of Good and Evil has appeared in man alone! And how are wolves always evil and rapacious, [Cf. p. xix. (in med.)] and lambs always illtreated and innocent, whereas men sometimes ravage like wolves and sometimes are illtreated like lambs? Who is he who arranged these things . . . and who [P.110, l.2.] is he who [gave] to creatures a 'bound Nature' so that creatures [have a fixed disposition], and to man gave an independent Will?