Why use "his" in association with the word "mankind"?

Solution 1:

The use of 'his' refers to each 'member' of mankind. Mankind is generally referred to in this way, i.e. as a group of individual people, as opposed to a single, monolithic body, which would be suggested by using 'its'.

Solution 2:

"his" (male pronoun) emphasises that it's a human.

"its" (neuter pronoun) would suggest a less human thing or entity.

Compare:

Jeremy the baker got used to his surroundings

and

Fido the dog got used to its surroundings

Solution 3:

This is an old conundrum. English has no proper neutral pronoun to either a male or female person. It is usually used to refer to inanimate or not-human things.

You could say:

The power to become habituated to one's surroundings is a marked characteristic of mankind.

But the usage of one as a pronoun has come to sound a bit pretentious. I believe there is a shift occurring in english where their is being used in the singular and becoming acceptable.

The power to become habituated to their surroundings is a marked characteristic of mankind.

Some would take it a step further.

The power to become habituated to their surroundings is a marked characteristic of humans.

But that's somehow less poetic. Humans seems more clinical, and mankind more familial and grandiose somehow.