I feel that I need to chime in overhere. What's been descibred in the quote is called superfetation. But mollies ain't superfetative livebearers. In non-superfetative livebearers, if one or a number of fry are born sooner than their siblings that actually belong to the same batch of fry is a result of two possible reasons. The most known reason is stress. The other reason is the amount of nutrients in each individual egg. When eggs are produced by a female, the eggs have the same amount of nutrients in general. This is called "pre-fertilization". But from time to time, the amount of nutrients can differ per egg. One batch of eggs will be fertilized at the same time. Doesn't mean that all eggs will be fertilized. If the sperm don't reach an egg, such an egg wil not be fertilized, of course. In case of more nutrients, once the egg gets fertilized, the developing embryo will develop and grow faster than its siblings of the same batch of eggs. This means also that such an embryo with more nutrients in its egg will be born sooner than its siblings. If one or a small number of molly fry are born sooner than the rest of its siblings, is not called superfetation.
Superfetation is a situation where a female carries multiple batches of embryos that were fertilized at different time periods which results in embryos in different stages of development. This causes births in an interval of 1-3 days over a time period of 10-14 days. But this only happens in superfetative livebearers such as Poeciliopsis, Heterandria, Neoheterandria, Phallichthys, certain Micropoecilia species and so on... But again not in mollies.
