By Marvin Payne (appearing as Mr. Bennet in BYU’s Pride and Prejudice)
[Most of this is edited from something John Hopfner wrote in a post to pemberley.com on 22 September 1996. My intrusions will appear in brackets. After apologizing for being neither a lawyer nor a legal historian, he writes in a lawyerly and historianly way:]
“The first thing to consider is the importance that ownership of land had, both in the England of Jane Austen’s time and in England for centuries previous to her day. Ownership of land wasn’t just an ornament to the family (in the way that a collection of paintings or a library might be considered an ornament). Land was what made a family part of the aristocracy or gentry. Ownership of land produced an income that was steady, predictable, and recurring. That income was what freed the family from the necessity to earn their living by daily effort. It freed them to secure and enjoy an education, to—as they chose—dabble in the arts and sciences, become involved in politics, or lead a life of idleness and refinement. This gave ownership of land a cachet that went beyond ownership of cash or movable goods. A landed estate was [called] The Patrimony—it conferred status in society, not just on one person for one generation, but on the family so long as it lasted.
“Members of the gentry and aristocracy [knew there were] two real dangers that threaten a landed estate:  dissipation by sale, if the head of the family at any point in time (a wastrel, say, or a foolish speculator) were to sell his land to raise funds, and then fritter away the sales proceeds; and  subdivision (if an estate were divided equally between all sons or children over several generations, then a single Patrimony, sufficient to make its holder a gentleman and member of the gentry, becomes a multitude of smaller patrimonies that, individually, don’t qualify his descendents for the same social status [because they generate insufficient income]).
“The result is that the whole family sinks into obscurity, which was held to be a bad thing. The answer to this problem is primogeniture [rights of the firstborn] among male heirs, which keeps The Patrimony itself intact and under the control of the head of the family in each generation—though at the cost of unfairness to other surviving children of the family head.
“If the family head dies without sons [one would think] the estate would be inherited equally by all the man’s daughters. If there were several daughters, they each would inherit an equal share, and the subdivision problem occurs. But even if the head of the family died leaving only one daughter, the daughter almost surely will marry — and at her death her heirs would be, presumably, the children she had with her husband. Which means that the ‘Bennet’ patrimony ceases to exist, and becomes part of the Darcy or Bingley estates (for example). [To prevent this, the common law made] provision to extend primogeniture to the entire male line, not just to the male sons of a given holder of a landed estate.” [Italics mine. This is where Mr. Collins comes in.]
Legal Aspects of Entails
The “common law” Hopfner mentions was a system of laws based not on what was legislated by representatives of the people, but on custom, tradition, and the tendency of court decisions on a particular subject over centuries. Of course, the elements of common law having to do with securing property through generations were shaped very early by the rich and powerful, so the sanctity of property ownership, beginning as custom and enforced by self-interested nobles, passed into law, which then reinforced custom.
The fact that an entail could be “broken” with the consent of the primary heir allows us an insight into Mr. Bennet’s vision of the future and apparent disdain for the past. Hopfner quotes this passage from chapter 50 in Pride and Prejudice:
“When first Mr. Bennet had married… of course, they were to have a son. This son was to join in cutting off the entail, as soon as he should be of age, and the widow and younger children would by that means be provided for.” [My italics.]
But he had no male heir, and Mr. Bennet hadn’t the authority to break the entail on his own, so Longbourn will go to Mr. Collins.
Attitudes to the Entail in Pride and Prejudice
Back to Hopfner:
“Jane Austen expected her readers to understand that it is no joke that if Mr. Bennet died, his wife and five daughters would have to leave Longbourn and live on the interest of £5,000 [$267,000 in today’s dollars], or a little more than £200 [$11,000] a year (because Mr. Bennet has been unable to save anything). Since Lydia alone costs Mr. Bennet about £90 [$5000] a year, it is obvious that their standard of living would drop; probably they would be partly dependent on the charity of the Gardiners, the Philipses, or even Mr. Collins. (After Jane Austen’s father died in 1805, Jane Austen and her mother and sister Cassandra needed an income of about £450 [$24,000], which had to be partly supplied by some of Jane Austen’s brothers.)
*These modern dollar equivalents are based on the fact that a British pound is thought to have purchased in 1810 what would require 34 pounds to buy today. A pound today equals about $1.57 US. This math is according to an anonymous guy on wiki.answers. I’ve rounded the figures upward promiscuously.
“Therefore Mrs. Bennet’s threat to Elizabeth that ‘If you go on refusing every offer of marriage, you will never get a husband—and I am sure I do not know who is to maintain you when your father is dead’ has some realism. This is the background against which Elizabeth and Jane are not desperate to be married to anyone with a good income.”
All this presupposes that finding a new way to do some work and make some money was beneath the dignity of someone who had inherited “gentry” status. We are to hope that after Mr. Bennet’s death, his widow and any unmarried (or ill-married) daughters will survive on the charity of Mr. and Mrs. Darcy and Mr. and Mrs. Bingley. (The Darcys make $533,000 annually, the Bingleys about half that.)