INTRODUCTION TO THE VERTEBRATES 237 



Here, as everywhere in Nature, we can see increase in com- 

 plexity, permitting greater division of labor, and this in turn 

 resulting in better adaptation and more perfect performance of 

 function. 



If we compare the protozoan to the man on the desert island, 

 then the sponge would represent a condition where there were 

 enough men (cells), so that one could do one thing and one, another. 

 It would be like a small village where one man could make all the 

 shoes, or do all the baking. 



In the hydra we find groups of similar cells (tissues) performing 



CROSS-SECTIONS 



VCRTE-BRATI 



FIG. 83. Note differences in location of similar organs of vertebrate 

 and invertebrate. 



a single function. This would correspond to the case where the 

 town had grown large enough so that many shoemakers or bakers 

 were required and they each worked together, as in a factory. 



Worms and higher forms, with their tissues grouped into organs, 

 would correspond to larger cities where many kinds of factories 

 were required to carry on the business of the still larger group of 

 people. 



COLLATERAL READING 



Applied Biology, Bigelow. pp. 417-419; Animal Studies, Jordan, 

 Kellogg and Heath, pp. 161-169; Economic Zoology, Kellogg and Doane, 

 pp. 237-240; Winners in Life's Race, Buckley, pp. 1-19; Animal Life, 

 Thompson, pp. 248-272; Comparative Zoology, Kingsley, pp. 127-156; Zo- 

 ology, Shipley and MacBride, p. 306; Elementary Zoology, Davenport, 

 pp. 289-297; Elementary Zoology, Galloway, pp. 274-280. 



