[tortoise]

Cancer, too, serves a purpose. Immortality is an evolutionary dead-end.

Non-sequitur. Evolution requires two processes, variation and selection. Immortality in any practical sense (e.g. you can still die in accidents, etc) would affect neither selection nor variation. Therefore, evolution would still occur even if we did not die of disease or aging.

[tortoise]

I should have said immortality does not STOP selection or variation, and therefore does not stop evolution. Doh!