156 DOCTRINE OF EVOLUTION 



sistent and reliable principles holding true at all times 

 and not intermittently, it would be difficult to order our 

 lives with confidence. In the next place, the general 

 principles of biology hold true for the structure and 

 physiology of the human species as they do for all other 

 living things. A human body is composed of eight 

 systems of organs, whose functions are identical with 

 the eight vital tasks of every other animal. All these 

 organs are made up of cells as ultimate vital units, 

 and the materials of which human cells are composed 

 belong to the class of substances called protoplasm. 

 Human protoplasm, like all other living materials, 

 must replenish itself, and respire and oxidize in obedi- 

 ence to biological laws that have been found to be uni- 

 form everywhere. Thus the human organism is no 

 more unique in fundamental organic respects than it is 

 apart from the world of physical processes and laws. 



How does the matter stand when the general struc- 

 tural plan of a human being is examined ? Is it entirely 

 different from everything else ? It is a fact of common 

 knowledge that the human body is supported by a 

 bony axis, the vertebral column, to which the skull is 

 articulated and to which also the skeletal framework of 

 the limbs is attached. These characteristics place man 

 inevitably among the so-called vertebrata ; he is cer- 

 tainly not an invertebrate, nor is the basic structure of 

 his body such that a third group, outside the inverte- 

 brata and vertebrata, can be made to include only the 

 single type — man. 



Passing now to the classes that make up the group of 

 vertebrates, we meet first the lampreys or cyclostomes 

 without jaws, and the others with jaws, such as the 



