28o THE HISTORY OF CKEATIOK 



CHAPTER XIL 



LAWS OF DEVELOPMENT OF ORGANIC TPIBES AND OF 

 INDIVIDUALS. PHYLOGENY AND ONTOGENY. 



La^n's of the Development of Mankind : Diflferentiation and Perfecting". 

 — Meclianical Cause of these two Fundamental Laws. — Progi'ess without 

 Differentiation, and Differentiation without Progress. — Origin of 

 Eudimentary 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. 



If man wishes to understand his position in nature, and 

 to comprehend as natural facts his relations to the 

 phenomena of the world cognisable by him, it is abso- 

 lutely necessary that he should compare human with extra- 

 human phenomena, and, above all, with animal phenomena. 

 We have already seen that the exceedingly important 

 physiological laws of Inheritance and Adaptation apply to 

 the human organism in the same manner as to the animal 

 and vegetable kingdoms, and in both cases interact with 

 one another. Consequently, natural selection in the struggle 



