Descriptive Biology 113 



haps as good as another. The conditions of the earth and its 

 climate under which the amphibians arose are considered in 

 the following chapter, but their earliest history is still a 

 blank. 



One of the most difficult, but withal interesting problems 

 of vertebrate morphology is the origin of the head. The 

 ancestral vertebrate lacked a head. How have its descendants 

 acquired one? This question involves in the first place one 

 fundamental to all evolution. Is an organ developed because 

 it is used, or is it used because it is developed? While this 

 question has ever been a terra incognita in biology we know 

 at least that form and function go hand in hand. This is 

 the corner stone of adaptation and survival. The animal is 

 a beautifully adjusted mechanism, in most cases built for 

 forward movement. The anterior end is the one which first 

 meets with changes in the animal's surroundings. Here food 

 is taken and danger encountered. Hence the development 

 at this end of a mouth and of special sense organs. In cor- 

 relation with the development of these parts, there is a 

 corresponding development of a brain or enlarged and spec- 

 ialized portion of the central nervous system, for receiving 

 the nerves coming from the organs of special sense, and for 

 sending out nerves controlling various parts of the body. En- 

 largements of the central nerve cord are not limited to the an- 

 terior end of the body. In the segmented worms and in arthro- 

 pods for example there are many such enlargements or ganglia, 

 one for each segment of the body, and when several of these 

 segments are combined into one, as occurs in the latter group, 

 notably in insects and crustaceans, the ganglia also fuse to 

 form compound structures. In the vertebrates, in corre- 

 spondence with the development of paired fins or limbs, there 

 are enlargements of the spinal cord opposite the latter, which 

 in some cases may even exceed the brain itself. Thus the 

 enormous dinosaur Stegosaurus had a "brain" in the sacral 

 region controlling the hind limbs which was larger than that 

 in the head. 



With the development of mouth, sense organs and brain 

 there is a corresponding development of parts to enclose, 

 protect and operate them. The development of jaws which 

 operate the mouth has already been briefly mentioned, as 

 have also the changes experienced by the gill arches in 

 the land vertebrates. The most difficult questions of head 

 development concern the brain case and especially the 

 muscles. 



The German anatomist Oken sought to solve the problem 

 of the brain case very simply by supposing that Nature had 

 enlarged and shaped three of the anterior vertebrae for this 



