Books on Amazon
Necessity of origin/organism/Kripke: (1972, 312ff, 1980, 110ff): thesis an organism could not have descended from another cell of origin as it actually did it - Simons pro - but the zygote is still no permanent essential part because it dies early - Phase sortal/McGinn: E.g. "child" - "adult" - accordingly also zygote.
Solution/Simons: it is essential for the organism, that it follows from sexual reproduction and that it has its zygote as an initial spurious part - unclear whether brain essential part (> brain transplant).
Necessary existence/Simons: is only possible with abstract objects: E.g. universals, numbers, etc. - Problem: if something exists necessarily, everything else depends on it.
Parts Oxford New York 1987