936 



LIFE : OUTLINES OF GENERAL BIOLOGY 



One sees at a glance the possibility of arranging the more active 

 types, like Infusorians, Ctenophores, Arthropods, Lancelets, Fishes, 

 Birds to the left and the more sluggish types, like Sporozoa, Echi- 

 noderms, Brachiopods, Polyzoa, Molluscs, Tunicates, Amphibians, 

 and Mammals to the right, while other great groups like Coelentera 

 and Reptiles illustrate both extremes, and others again like Rhizo- 

 pods and "Worms" are, as it were, on a via media, neither very 

 active nor very sluggish, but a compromise between the more 

 pronounced rights and lefts. And in a class like that of the Polych?et 



Fig. 158. 



A Survey of Invertebrate or Backboneless Animals, i, A parasitic protozoon, 

 a gregarine; 2, an amoeba; 3, an infusorian, Paramoecium; 4, a very- 

 simple vase-like sponge; 5, a simple polyplike hydra; 6, a medusoid or 

 swimming-bell ; 7, a jellyfish or medusa ; 8, a higher polyp or sea-anemone ; 

 9, a liver-fluke or trematode; 10, a brittle-star, one of the echinoderms; 

 II, a leech; 12, an earthworm; 13, a prawnlike crustacean; 14, Peri- 

 patus; 15, an insect; 16, a spider; 17, a snail. 



Annelids, the active "errant" types like Nereis are contrasted with 

 the passive sedentary types like Serpula and the coral-like Filigrana, 

 just as Lizards with Tortoises at a higher level. 



Let us suppose for the moment that this grouping is neither 

 fanciful, nor the projection of a personal theory, what contribution 

 to biology does it make? It suggests that the main mode of branching 

 of the genealogical tree has been dichotomy after dichotomy, 

 sometimes with intermediate types between the divergent lines, 

 and that the fimdamental reason for the divergence is simply that 

 these are the great metabolic alternatives. In other words, the most 

 frequent of all variational alternatives is to increase or decrease the 





