According to the author's opinion, there are the following respectasons: firstly, the high latitude areas hasve longer nights, so theresulting in northerners can sleeping longer, while the and southerners do the opposite.sleeping less. (assuming sleeping longer results in taller height)

Secondly, the northern area produces wheat, and wheatwhich has a longer growing period than rice, and thuso wheat is more nutritious. (assuming a longer growing period equates to greater nutrition)

Therefore, northerners mainly eat wheat so that they can b, in effect, are taller than the southerners who mainly eat rice.

Thirdly, the north is colder than the south, so people there need to eat more (andthey mainly eat beef, mutton and milk) to maintain their fat stores and calories to keep warm and this leads that they are, which makes them taller and bigger (assuming eating more of these products equates to being taller and big, while people in the South not only don't need fat and calories to keep warm because of the hot weather there, on the contrary, they neger). In contrast, because of the hot weather in the South, people do not need as much dietary fat and calories to keep warm and rather, would benefit from increased heat dissipation, andsomething a short body is very good for that.

I would try to make shorter sentences to keep your thoughts more clear.
This is an interesting theory but there are a lot of assumptions. For example, I thought a taller stature would mean a larger surface area for heat dissipation (as opposed to a shorter stature)?

Their bodies evolved naturally to adoapt to the local climate.