One of the strangest opinions related in Aristotle, is the view he ascribes to Empedocles in Physics II.8.  In context, Empedocles is invoked after Aristotle asks why we should not treat every natural occurrence like the rain, that is, as a process which does not occur for the sake of something, and applying this line of explanation to everything in nature, also say, for example, that teeth came to be in such a way as to be merely coincidentally felicitous for animal chewing.  Of course, on the other hand, things can also turn out coincidentally poor for animals as well, and such is the case when Aristotle relates the monstrous suggestion of Empedocles:

ὅπου μὲν οὖν ἅπαντα συνέβη ὥσπερ κἂν εἰ ἕνεκά του ἐγί-
γνετο, ταῦτα μὲν ἐσώθη ἀπὸ τοῦ αὐτομάτου συστάντα ἐπι-    (30)
τηδείως· ὅσα δὲ μὴ οὕτως, ἀπώλετο καὶ ἀπόλλυται, κα-
θάπερ Ἐμπεδοκλῆς λέγει τὰ βουγενῆ ἀνδρόπρῳρα.

So when all turned out just as if they had come to be for something [ἕνεκά του, i.e., final cause], then the things, suitably constituted [συστάντα ἐπιτηδείως] as an automatic outcome, survived; when not, they died, and die, as Empedocles says of the man-headed calves. (Trans. Charlton, Physics II.8, 198b29-32).

It was perhaps because of Aristotle’s own example of teeth coming to be fortuitously arranged that prompts his introduction of the man-headed calves, whose dentition was not favorable to the kind of food suitable for a bovine digestive system.  Whatever was the cause of the demise of the poor man-headed ox (were Aristotle to grant that it ever existed), however, we can infer that due to some mismatch of parts, the animal was unable to survive.  Yet, remember that the man-headed ox is an interlude to the discussion of rain, to which Aristotle now wishes to return, adding an important qualification to emphasize how in fact rain is determined by a final cause.       

ὁ μὲν   (32)
οὖν λόγος, ᾧ ἄν τις ἀπορήσειεν, οὗτος, καὶ εἴ τις ἄλλος
τοιοῦτός ἐστιν· ἀδύνατον δὲ τοῦτον ἔχειν τὸν τρόπον. ταῦτα
μὲν γὰρ καὶ πάντα τὰ φύσει ἢ αἰεὶ οὕτω γίγνεται ἢ ὡς ἐπὶ   (35)
τὸ πολύ, τῶν δ’ ἀπὸ τύχης καὶ τοῦ αὐτομάτου οὐδέν. οὐ
(199a) γὰρ ἀπὸ τύχης οὐδ’ ἀπὸ συμπτώματος δοκεῖ ὕειν πολλάκις
τοῦ χειμῶνος, ἀλλ’ ἐὰν ὑπὸ κύνα· οὐδὲ καύματα ὑπὸ κύνα,
ἀλλ’ ἂν χειμῶνος.

This [i.e. the biologically advantageous occurring by coincidence], or something like it, is the account which might give us pause.  It is impossible, however, that this should be how things are.  The things mentioned, and all things which are due to nature, come to be as they do always (αἰεὶ) or for the most part (ὡς ἐπὶ τὸ πολύ) and nothing which is the outcome of luck or an automatic outcome does that.  We do not think that it is the outcome of luck or coincidence that there is a lot of rain winter, but only if there is a lot of rain in August; nor that there are heatwaves in August, but only if there is a heatwave in winter. (Trans. Charlton, Physics II.8, 198b32-199a3).

Here Aristotle adds that when things occur with a level of considerable frequency, they cannot be attributed to luck or the automatic.  When rain comes about in the winter (as opposed to a meteorologically unusual time), it is the result of a final cause.  Alan Code points out how this teleological explanation of rain can be fruitfully paralleled to Aristotle’s other example of teeth, “So too we can distinguish the formation of a front tooth simpliciter from the formation of a front tooth during the development of a human, and see that the latter is not coincidentally connected with the suitability of the tooth for biting”  (132).  This is saying, in a way suitably technical for Aristotle, that teeth are not things that come about out of the blue, as if a set of marbles or flowers were just as likely candidates to have filled the mouth of a lion as were canines and molars.  Rain happens frequently or for the most part in the winter, while in lions frequently or for the most part teeth come about during its process of maturation.  These teeth, considered as parts, of the animal must be considered with an eye to the form, that is, the final cause of the animal, if we are to make any sense of why they happen to come about with regularity.

A passage that might be pressed into service on this point of the poverty of material explanation is in Physics II.9 as Aristotle us tells how a real rube might suppose a wooden city wall is built.  On this person’s misunderstanding, what happens is that the foundation of the wall, the stones and gravel, sink down into the earth because they are heaviest, then the earth, a little lighter, comes to rest on top of this, while finally, lightest of all, the wooden posts of the fence itself surmount the earth.  His criticism of this understanding of a city wall is that although it is necessary for a city wall to have these three parts, they are nevertheless nothing more than a material cause of the wall.  They do not tell us that the wall is for the protection and preservation of certain things (ἕνεκα τοῦ κρύπτειν ἄττα καὶ σώζειν) (Physics II.9, 200a6-7).  Aristotle’s explanation helps to draw out the fact that the form of something can also be referred to as its account or definition (λόγος).  Within the definition of a city wall, of course, is the notion that it is for the protection and preservation of certain things, and definition to a greater degree in fact, than whether it is made of wood, chain links, or concrete, gives us a meaningful explanation of the wall.  In the ox-headed man example, then, the reason a man has a man-head is because the form of the man dictates that he has such a part as a man, not, as Empedocles might erroneously assert, that the reason a man has a man-head is because, of the parts that happened to come about, one was a man-head.  Similarly, if Empedocles’ ox-man was more than merely fanciful, but based on observation of biological deformation, then the rare irregularity of these monsters presents the same case: the regularity of the occurance of non-deformed boys and oxen testifies to the existence of a final cause, their form.