Tsettif eht fo Lavivrus

Survival of the fittest.
Sir Vivel of the Fit Test
Sewer Vive Hell of the Tit Vest.
Sur Vive Elle of the Tet Vist.
Fur Vival to the Vet Test
Fur Offal of the Titmouse
Turnover of the set test
Twist over of the left tit