The letters of the word 'MANKIND' are written in all possible orders and arranged in serial order as in an English dictionary. Then the serial number of the word 'MANKIND' is ______.

We need to find the serial number of the word "MANKIND" when all permutations of its letters are arranged in dictionary order.

First, we list the letters in alphabetical order. MANKIND has letters: M, A, N, K, I, N, D. Sorted alphabetically, they become A, D, I, K, M, N, N; note that N appears twice.

Since each word starting with a letter before M (namely A, D, I, K) leaves six letters with two Ns, each such prefix yields $$\dfrac{6!}{2!} = 360$$ arrangements, giving a subtotal of $$4\times 360 = 1440$$ words before those beginning with M.

When the first letter is M, no letter precedes A, so no words begin with MA that come before "MANKIND".

Once MA is fixed, the remaining letters are D, I, K, N, N. Because D, I, and K all come before N, each choice leads to $$\dfrac{4!}{2!} = 12$$ permutations, totaling $$3\times 12 = 36$$ words before MAN.

Fixing MAN leaves D, I, K, N; among these, D and I precede K, and each gives $$3! = 6$$ permutations, so there are $$2\times 6 = 12$$ words before MANK.

With MANK as prefix, the remaining letters are D, I, N, and only D comes before I, giving $$2! = 2$$ words before MANKI.

When the prefix is MANKI, the leftover letters are D and N, and D precedes N, yielding $$1! = 1$$ word before MANKIN.

Finally, for words starting with MANKIN, the only remaining letter is D, so there are no further words before MANKIND.

The +1 in the sum accounts for the word "MANKIND" itself, so it occupies the position $$1492$$ in the dictionary order.