I don't understand this part of the book about this example in the probability theory book:
Let $C$ be a set in $n$-dimensional space and let $$Q(C)=\int_C dx_1\,dx_2\cdots dx_n.$$
If $C=\left\{(x_1,x_2,\ldots,x_n): 0\le x_1\le x_2\le \cdots \le x_n\le 1\right\},$ then
\begin{align} Q(C)&=\int_0^1\int_0^{x_n}\cdots\int_0^{x_3}\int_0^{x_2} dx_1\,dx_2\cdots dx_{n-1}\,dx_n \\&=\frac1{n!}\,, \end{align} where $n!=n(n-1)\cdots 3\cdot2\cdot 1$.
I don't understand in the $Q(C)$ part. Why is the integral from $x_n$ to $0$ written first, and why is it $dx_1\, dx_2\cdots dx_{n-1} \,dx_n$ ?