If I'm not mistakened, white tigers are a product of a recessive gene. Thus, if a white tiger existed in the wild, it would easily be hunted out and would not survive as well as the dominant coloreds (the normal red/orange tiger). Humans, on the other hand, are fascinated with rare colors and so circuses and unreputable zoos started breeding white tigers. In fact, our zoo does not accept white tigers because they are inbred and have many unnatural disorders. Thus, I wouldn't agree it is necessary to continue their lineage. However, you're right in that many many many species of fauna AND flora would be extinct without the help of zoos, breeding programs, wildlife refuges, and reintroduction efforts. In this day and age where we are continually pushing species of plants and animals (even the creepy crawly insects) to the brink of extinction, I think zoos are the lesser of the two evils. Continue a lineage with the faint hope of reintroduction? Or stand by and watch them slowly die out?