It's ok to murder human infants, but it's not okay to kill an animal.
If a child were well-behaved, it was safe, of course. But disobedient children could be turned in by their parents. They'd be given a math test, and if they didn't do well, they would be "eliminated".
Eventually, a High School Math Teacher climbs into the Child Collection Van and professes that he doesn't really understand algebra that well and asks them what they will do about it.
The point: all such yardsticks are arbitrary and wrong. Human life is sacred, and it is not hard to identify "human life". Only evil people pretend that it's an intellectual challenge.
Just did a quick Google search on him ... oh brother! From The International Vegetarian Union web site ...
Peter Singer is now a Professor at Princeton University, USA. He was formerly Professor of Philosophy and Director of the Centre for Human Bioethics at Monash University, Melbourne, Australia. He is the author of Animal Liberation, which can be considered the Bible of the animal rights movement. Recently, he has been instrumental in the formation of the The Great Ape Project, which seeks to extend personhood and legal rights to the Geat Apes.