48 
MAURICE G. MEHL 
tures illustrating the main types of vertebrates that the writer 
was led in an attempt to supply the materials by means of charts. 
It was recognized that even when certain structures were well 
described, such illustrations as were available were often in- 
adequate and gave the student little more than an idea of the 
shape of the bones. Even to gain this it was usually necessary 
to work through a great mass of literature, often not available. 
To work out a chart for each form or group with which it was 
desired to become familiar would mean an endless task. Fur- 
thermore, such charts would not have the vital interest which 
the preparator gains as he ^Vorks out’^ and mounts a skeleton. 
It was thought that could the student be furnished with a set 
of conventional ^^paper bones’^ from which he could construct 
skeletons at will, much of the difficulty of teaching the verte- 
brate skeleton would be eliminated. 
Some time ago the writer conceived the idea of compiling a 
guide chart on which were designated the elements in the primi- 
tive skull together with the conspicuous openings in their proper 
relations. Working oh the principle that organic evolution in- 
volves only the loss of parts or the modification of existing ele- 
ments, the student was expected to construct any skull from the 
base by discarding or utilizing the lines bounding the primitive 
elements. 
The results of this experiment were very gratifying. While 
the guide chart had been compiled for the sole purpose of supply- 
ing laboratory materials, it was found that it answered other 
purposes as well. In the first place the student came to realize 
that it was the elements and openings and their relations in the 
skull that were all important ; the size and shape were of second- 
ary consideration. The use of the same size and shape of 
skull to show the structure of various types seemed to give a new 
meaning to the word ^ ^structure” : the student observed that 
between the largest and the smallest dinosaur skulls there were 
fewer differences than between many skulls that in a hasty 
glance seemed very similar. 
Again, he came to a realization, as never before, that every 
form, no matter how complicated, was merely a modification, 
