I never realized this before but according to the annotation in my book, "Elizabeth's comment is a subtle reference to the important writer on the picturesque, William Gilpin, an author Jane Austen admired. Gilpin used cattle when discussing his ideas of the picturesque, for cattle were frequent subject of landscapes; one of his main principles was that three was the ideal number of cattle to form a picturesque grouping, and that the effect would be ruined by introducing a fourth..."

I have searched around for a way to scan in the picture that Gilpin included to demonstrate his point but could not find one. (I thought that I had seen somewhere on this site, maybe a remark from Julie or something, that this was possible if you had the technology which I'm not sure I have anyway) and I looked around on the internet for it to include as a link but was unsuccessful. It is from his book---Observations, on Several Parts of England, particularly the Mountains and Lakes of Cumberland and Westmoreland, relative chiefly to Picturesque Beauty, made in the Year 1772, Vol. II (London, 1808), p.254. Perhaps someone else will have more success.

Anyway, as soon as I saw this picture, Lizzie's remark took me to a whole new level of understanding. It is truly clever and too funny for words once you see it.