To me the whole key to the meaning of this is the meaning of the unfamiliar (to me) word "taxcollars".
I found the following passage in a travel guide from 1875:
One day when walking in the principal street of Klausenburg I heard a
great barking amongst the dogs, of which there were some dozen
following a closed van. On inquiry I found that once a-week the
authorities send round to see if there are any dogs at large without
the regulation tax-collar. If any such vagabonds are found they are
consigned to the covered cart, and are forthwith shot.
So it looks like, in at least this one sense, a contemporary to Clemens was using "tax-collar" to indicate their equivalent of the pet registration tags we put on pet collars today to indicate their owners, and show that they've been inoculated against dangerous diseases.
"Trust" was the word used around the turn of the 20th century to indicate large powerful corporations.
Looking at the rest of the context here, he is in fact talking about dogs in the previous sentence. So I'm guessing that even though the word "herd" is used, its is being used somewhat ironically to indicate a pack of dogs being treated more like a herd of livestock.
So basically, this is a clever (hopefully humorous) way of saying that the mother is really good with words, and also that Laboratory is a better word than scientist.