Let $n$ be a non-zero natural integer and let $X_{1}, \ldots, X_{n}$ be mutually independent random variables following Poisson distributions with respective parameters $\lambda_{1}, \ldots, \lambda_{n}$. Show that $X_{1} + \cdots + X_{n}$ follows a Poisson distribution with parameter $\lambda_{1} + \cdots + \lambda_{n}$.