This was posted over on Vridar, which is itself apparently an argument made by Mike Goulder. Regardless of who made the argument or whether he actually said it, it looks like it is a valid observation.
Everyone knows about the metaphor of “wolf in sheep’s clothing”, but this is only one out of 10 such animal allusions in the Gospels. The full list:
Give not what is holy to dogs, and cast not your pearls before swine. (Matt 7.6) *
Or he asks for fish, will he give him a snake? (Matt 7.10) *
Who comes to you in sheep’s clothing, but inward are ravening wolves. (Matt 7.15) *
Foxes have holes, and birds of the air have nests. (Matt 8.20)
I am sending you out as sheep in the midst of wolves. (Matt 10.15)
So be as wise as serpents and innocent as doves. (Matt 10.16) *
You strain at a gnat but swallow a camel. (Matt 23.24) *
You snakes, you brood of vipers! (Matt 23.33)
As a hen gathers her chicks under her wings. (Matt 23.37)
As a shepherd separates the sheep from the goats. (Matt 25.32) *
Notice something here? These are all in Matthew. None of them are in Mark or John. The only other place that they show up in is in Luke. Specifically, the “double tradition” material of Luke.
So there are three explanations for this. One, Matt simply expanded on the animal imagery in Q (like Matt 7.10). However, all of the verses that I put asterisks by are the ones that are only found in Matt; the double tradition only has four out of the 10 listed here as animal imagery.
Another explanation is that Matt invented all of the animal imagery and Luke used Matt as a source. This would explain why all of the animal imagery isn’t found in both Matt and Luke if it was originally from Q. Luke didn’t use Matt in its entirety. This is the one that I favor, obviously, but the previous argument still makes sense. Matt could have just expanded on the animal imagery in Q.
A third possibility is that Luke invented the animal imagery and Matt expanded on Luke. In and of itself, this seems just as likely as Matt expanding on Q. That is, if we didn’t have any other background information, Matt expanding on Q and Matt expanding on Luke seem equally as probable (but we do have background information so it seems slightly less likely that Luke invented the animal imagery).
But it is still a matter of probability. If Q exists, then Matt more than doubled the animal allegories. If Q didn’t exist and Luke is using Matt, then Luke took out more than double the animal imagery. Which is more probable? Adding to stuff, or subtracting stuff? In the Synoptic tradition as a whole, the authors were more likely to add stuff than to subtract stuff. Matt in particular expands on Mark.
So taking the redaction trend of Matt, it would seem more likely that Matt just expanded on the animal allegories in Q instead of Luke using Matt as a source.