What are the differences between ionic bonds and covalent bonds?

The main difference between ionic and covalent bonds is the sharing of electron pairs and atoms. In covalent bonds, atoms are attracted towards each other while in ionic bonds; electron pairs are shared between atoms.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".

the endomembrane system i think