Many animals die in shelters, every single day.  If a shelter/SPCA is over-crowded already, animals may have to be put down to make room, or be put down on their way in. Besides the great action of adopting one of these poor souls, there is something else you could do. CONSIDER FOSTERING. Your local shelter, I'm sure, can tell you how to get started. It WILL save lives. It's only temporary, and the experience will last you a lifetime. Pledge to try fostering, or at the very least, look into it.
*Picture at left is of Mama cat Ruby, her broad was my first ever foster. She, and her kittens, have since all found great stable homes!*