THE HISTORY OF ANIMAL LIFE 417 



extrusions of the cell substance, the pseudopodia; the white blood 

 cells of Man move in the same fashion. Infusoria move by means 

 of cilia; in Man the lining of the respiratory passages is made up 

 of cells that have cilia on their exposed surfaces, which by con- 

 stant beating maintain a flow of the overlying fluid. Masti- 

 gophora propel themselves through the water by the motions of 

 flagella; the sperm cells of most animals have a similar mechanism 

 of locomotion. 



Histological preparations of various glands in vertebrates show 

 striking resemblances in the nature and arrangement of the cells; 

 liver tissue, for example, is easily distinguished, but the particular 

 vertebrate from which the preparation was made cannot so easily be 

 identified from the material. Not only do such tissues appear simi- 

 lar in preparations, but their products have the same physiological 

 action throughout the vertebrate series. Therapeutic preparations 

 from the thyroid, ovaries, pituitary, and other endocrine glands of 

 lower vertebrates are effectively employed in the treatment of hu- 

 man patients. Moreover, the nearer the phylogenetic relationship the 

 more nearly alike is the response of the blood to certain chemical 

 tests. And in the phylogenetic series some Mollusca, the Annelida 

 and all Vertebrata including Man utilize haemoglobin as the chemi- 

 cal agent for the transfer of oxygen by the blood. 



The foregoing pages have given merely the high lights of the 

 various sources of evidence upon which the concept of evolution 

 is based. It is not feasible to cite here more facts that are made 

 rational by this interpretation; hundreds of volumes are required 

 to cover the evidence now at hand. Familiarity with only that por- 

 tion of the evidence cited convinces one that the facts show beyond 

 a doubt the paths of origin of present-day living forms, although 

 the exact direction of some paths is obscure in the great past. More- 

 over, no evidence is at hand which points to some conclusion other 

 than the principle of evolution. 



