A sequence $\left\{ a _ { n } \right\}$ has $a _ { 1 } = 2$ and satisfies for all natural numbers $n$:
$$a _ { n + 1 } = \left\{ \begin{array} { c c }
\frac { a _ { n } } { 2 - 3 a _ { n } } & ( \text{when } n \text{ is odd} ) \\
1 + a _ { n } & ( \text{when } n \text{ is even} )
\end{array} \right.$$
What is the value of $\sum _ { n = 1 } ^ { 40 } a _ { n }$? [3 points]\\
(1) 30\\
(2) 35\\
(3) 40\\
(4) 45\\
(5) 50