However, the poem seems to be contradictory-if I have interpreted it right. Initially it mentions that natural selection proceeds by necessity. Later it claims that evolution is purposeless.

The latter is actually true. There is no purpose to evolution. Evolution does not proceed in the direction that leads to anything ‘better’. A common misconception is of ‘mother nature’. There is no mother nature guiding this process.

Lets say tomorrow a group of humans are able to reproduce more efficiently by acquiring a trait which we conventionally perceive as ‘bad’. Whether it is ‘bad’ or ‘good’, by the very fact that they can reproduce faster, their progeny will overwhelm the numbers of our progeny and eventually our progeny will be wiped out.