HIGHWAY OF ANIMAL EVOLUTION 221 



litical affairs, sudden eruptions of growth ensue, ac- 

 companied by still other readjustments which neces- 

 sarily follow in their train; and great classes, orders, 

 or families of animals may arise therefrom, differing 

 among themselves in countless details, but all of them 

 upbuilt on some particular modification of this basic 

 architectural plan. 



In some of the lower forms of bilateral animals 

 (clams, oysters, and certain worms) the entire body 

 may consist of but a single metamere, and although, in 

 some instances, such animals may become very volumi- 

 nous, no one-compartment animal, highly organized, 

 or with great freedom of movement, has ever made its 

 appearance. 



There are also certain worm-like animals (anne- 

 lids) whose body may consist of a long array of meta- 

 meres, all very much alike, and each one supplied with 

 a complete set of the more essential vital organs, such 

 as eyes, legs, kidneys, germ-cells, ganglia, etc. No ani- 

 mal of this kind ever attained a high grade of organic 

 efficiency, no doubt because of the very multiplicity of 

 parts doing essentially the same things, and because one 

 part either does not, or is unable efficiently to serve the 

 other. 



On the other hand, in the long series of animals 

 which lead more directly up to man, figs. 7, 8, the body 

 may consist of from two or three metameres to a hun- 

 dred or more ; but with each addition, there is generally 

 a marked local diversity of parts in the older, more 

 cephalic ones, and better centralization of organic serv- 

 ices. To follow these changes, even in the barest out- 

 line, one must make a careful comparative study of the 

 anatomy, physiology, and paleontology of all those 



