Usbek to the same
You are seeking the reason why the earth is less peopled than it used to be, and if you look closely, you will see that the great difference comes from the difference that has occurred place in behavior.
Ever since the Christian and Muhammadan religions divided up the Roman world, things have much changed : these two religions are by no means as favorable to the propagation of the species as the religion of those masters of the world.
In the latter, polygamy was forbidden, and in that it had a very great advantage over the Muhammadan religion ; it permitted divorce, which gave it another, not less considerable, advantage over Christianity.
I find nothing so contradictory as this plurality of wives allowed by the holy Qur’an and the command to satisfy them commanded in the same book. See your wives, says the Prophet, because you are as necessary to them as their garments, and they are as necessary to you as yours. That is a precept that makes the life of a true Muslim very demanding. Must not he who has the four wives established by the Law, and only an equal number of concubines and slaves, be weighed down by so many garments ? 
Your wives are your plowing, says the Prophet again ; go therefore to your plowing, do your souls good, and one day you shall find it. 
I regard a good Muslim as an athlete, fated to compete continually, but who, soon weak and weighted down by his initial fatigues, languishes in the very field of victory, and finds himself, so to speak, buried under his own triumphs.
Nature always acts deliberately, and so to speak economically ; her operations are never violent ; even in her productions she wants temperance ; she never proceeds but with method and measure. If she is forced, she soon falls into languor ; she uses what strength she has remaining to preserve herself, utterly losing her productive virtue and reproductive potency.
It is in this state of breakdown that this multiplicity of wives always puts us, more likely to exhaust us than to satisfy us. It is quite usual in Persia to see a man in a prodigious seraglio with a very small number of children ; even these children are for the most part weak and unhealthy, and affected by their father’s languor.
This is not all : the wives, obliged to a forced continence, need men to guard them who can only be eunuchs  : religion, jealousy, and even reason do not allow others to approach them. There must be a large number of these guardians, either to maintain tranquillity within, amidst the wars which these wives are continually wage against each other ; or to prevent attempts from without. Thus a man who has ten wives or concubines needs at least as many eunuchs to guard them. But what a loss for society, this large number of men dead at birth ! What depopulation must necessarily ensue ! 
Slave girls who are in the seraglio to serve with the eunuchs : this large number of women grow old there almost all in a pathetic virginity  ; they cannot marry as long as they remain there, and their mistresses once accustomed to them, almost never let them go.
That is how one single man alone occupies so many subjects of both sexes for his pleasures, makes them die to the state, and renders them useless for the propagation of the species.
Constantinople and Isfahan are the capitals of the two greatest empires on earth : that is where everything must lead, and where peoples drawn in a thousand ways come from everywhere. And yet they perish by themselves, and they would soon be destroyed if the sovereigns did not in almost every century bring entire nations to repopulate them. I shall complete this topic in another letter.
Paris this 13th day of the moon of Chahban 1718