Chapter XII

LAWS OF DEVELOPMENT OF ORGANIC TRIBES AND OF INDIVIDUALS.
PHYLOGENY AND ONTOGENY.

Laws of the Development of Mankind: Differentiation and
Perfecting.—Mechanical Cause of these two Fundamental
Laws.—Progress without Differentiation, and
Differentiation without Progress.—Origin of Rudimentary
Organs by Non-use and Discontinuance of
Habit.—Ontogenesis, or Individual Development of
Organisms.—Its General Importance.—Ontogeny, or the
Individual History of Development of Vertebrate Animals,
including Man.—The Fructification of the Egg.—Formation
of the Three Germ Layers.—History of the Development of
the Central Nervous System, of the Extremities, of the
Branchial Arches, and of the Tail of Vertebrate
Animals.—Causal Connection and Parallelism of Ontogenesis
and Phylogenesis, that is, of the Development of
Individuals and Tribes.—Causal Connection of the
Parallelism of Phylogenesis and of Systematic
Development.—Parallelism of the three Organic Series of
Development 280