In Maseches Beitzah 7a the gemara says that any animal that mates by day will give birth at day-this is talking about a chicken ,and any animal that mates by night gives birth at night, this is talking about a bat. Any being that mates by day or night will give birth either by day or night-this refers to man and alike beings.
The gemara goes on to say that if a person checks a chickens nest erev yom tov and doesn't see an egg,but then after yom tov starts he checks again and finds the egg (fertilized egg see gemara inside), as long as it was before daybreak the egg is permissible since we say that it was there from before yom tov since a chicken doesn't give birth at night.
My question is was this a scientific belief in those days or a halachic reality, and would we say this halacha today? (I don't know the zoology aspect of this case.)