I think the author makes a distinction between the mechanical view of evolution, which he accepts is correct, and the emotive view which he acknowledges is neither accurate or logical, but stems from a desire to attribute will to nature. Obviously fish don’t will legs on themselves- he’s using this to introduce the spiritual idea of Will as a concept, a will-to-life not necessarily rooted in anything mechanical. I see it less as anything tangible and more as an idea used to explain Schopenhauer’s argument. Tbh this part didn’t make a ton of sense to me either.

As for the leap connecting will and violence, he’s saying possessing a desire to live, in a natural setting with limited resources, inherently puts you in competition with other beings with a will-to-live, leading to violence and suffering. Optimism (the author argues) denies this reality, while pessimism accepts it- it’s not miserable so much as realistic, and is the starting point for the “enlightenment” Schopenhauer talks about. Once you accept the reality that suffering is caused by Will, one can begin the journey to temper their Will and reduce the suffering it causes.

That’s how I understood it, at least.