152 THE ORIGIN AND EVOLUTION OF LIFE 



Taking the whole history of vertebrate Hfe from the beginning, 

 we observe that every prolonged, old adaptive phase in a sim- 

 ilar habitat becomes impressed in the hereditary characters of 

 the chromatin. Throughout the development of new adaptive 

 phases the chromatin always retains more or less potentiality 

 of repeating the embryonic, immature, and more rarely some 

 of the mature structures of older adaptive phases in the older 

 environments. This is the basis of the law of ancestral repeti- 

 tion, formulated by Louis Agassiz and developed by Haeckel 

 and Hyatt, which dominated biological thought during thirty 

 years of the nineteenth century (1865-1895). It yielded with 

 more or less success a highly speculative solution of the ances- 

 tral form history of the vertebrates, through the study of em- 

 bryonic development and comparative anatomy, long before 

 the actual lines of evolutionary descent were determined 

 through palaeontology. 



Laws of Form Evolution in Adaptation to the Mechani- 

 cal AND PhYSICOCHEMICAL ACTIONS, REACTIONS, AND 



Interactions of Locomotion, Offense and 

 Defense, and Reproduction 



The form evolution of the back-boned animals, beginning 

 with the pro-fishes of Cambrian and pre-Cambrian time, ex- 

 tends over a period estimated at not less than 30,000,000 

 years. The supremely adaptable vertebrate body type be- 

 gins to dominate the living world, overcoming one mechan- 

 ical difficulty after another as it passes through the habitat 

 zones of water, land, and air. Adaptations in the motions 

 necessary for the capture, storage, and release of plant and 

 animal energy continue to control the form of the body and 

 of its appendages, but simultaneously the organism through me- 

 chanical and chemical means protects itself either offensively 



