THE LAWS OF ADAPTATION 



153 



or defensively and also adapts 

 itself to reproduce and protect 

 its kind, according to Darwin's 

 original conception of the strug- 

 gle for existence as involving 

 both the life of the individual 

 and the life of its progeny. 

 Among all defenseless forms 

 either speed or chemical or elec- 

 trical protection is a prime 

 necessity, while all heavily ar- 

 mored forms gradually aban- 

 don mobility. As among the 

 Invertebrata, calcium carbon- 

 ate and phosphate and various 

 compounds of keratin and chi- 

 tin are the chief chemical ma- 

 terials of defensive armature. 



Locomotion, as distinguished 

 from that in all invertebrates, 

 is in an elongate body stiffened 

 by a central axis, hence the 

 name chordatc or Chord ata for 

 the vertebrate division. The 

 evolution of the cartilaginous 

 skeletal supports (endoskeleton) 

 and of the limbs is generally 

 from the centre of the body 

 toward the periphery, the evolu- 

 tion of the epidermal defensive 

 armature (exoskeleton) is from 

 the periphery toward the centre. 



Fio. 40. Total Geologic Time Scale, 

 Estimated at Sixty Million Years. 



These estimates are based upon the 

 relative thickness of the pre-Cambrian 

 and post-Cambrian rocks. Prepared 

 by the author and C. A. Reeds after 

 the time estimates of Walcott and 

 Schuchert. 



