Serpents, poisonous, [50].
Sexual characters of apes, [49].
Sexual selection, [48].
Sharks, [83].
Shell-fish, beauty of, [54].
Shells of oysters, [88], [98].
Shielded grasshopper, [89].
Silurian strata, [140], [142].
Simultaneous modifications, [57].
Sirenia, [42]