Is it really that difficult to see how some creatures are more valuable than others? It's useless to compare cats and rabbits because they are basically equivalent. The cancer and the person are not. The cat and bunny are not equal to lettuce either. These are simple comparisons that we don't have to think about. The distinction is of one organism being of a more complex and therefore higher order. There is a very real natural heirarchy that has many levels. Some creatures share a level with others (cats and dogs..) and some are far above others (humans vs. fruit flies). If you think all living things have equal rights to exist then I must wonder what it is you eat.
I feel a little light headed... maybe you should drive...