Pre-established harmony, [25].

Preformation, [53].

Protective colorings, [14] et seq.

Protogonius, [22].

Pseudocræa, [52].

Qualitative modifications, [46].

Quantitative changes, [46]-[47].

Retrogressive development, [38].

Round-worms, eggs of, [28].

Roux, Wilhelm, [29], [39], [65], [66].