A study of rabbits in the 1940s convinced many biologists that parthenogenesis—reproduction without fertilization of an egg—sometimes occurs in mammals.
Conclusion (thus)
There must be something about mammalian chromosomes that precluded parthenogenesis.
Evidence
Parthenogenesis occurs in a wide variety of nonmammalian vertebrates, no studies have yet demonstrated parthenogenesis in mammals, and the one study that supposedly did was shown to have flawed methods.
Evaluation
This is an (unusually hard) example of a famous flaw called Absence of Evidence (or Unproven vs. Untrue). The idea here is that it’s flawed to think that just because no one has ever successfully proven X that X is therefore false. We could otherwise argue, , and the faithful could equally respond, . When we hear the author saying that someone’s attempt to prove an idea failed (whether it’s because of shaky methodology, failure of others to replicate, reliance on a dubious assumption, etc.), then we should be left in a state of doubt about that idea. Maybe it’s true, maybe it’s not.
That’s how the author should feel about mammalian parthenogenesis. Instead, the author accepts the pessimistic notion that parthenogenesis doesn’t occur in mammals, and her conclusion goes on to speculate why there isn’t.
Goal
We either want an answer that says, , or we could look for an answer that targets the conclusion and says,