THE HISTORY OF ANIMAL LIFE 409 



trol of the will, whereas in mammals of lower type these muscles 

 are well developed and under control. More than one hundred such 

 vestiges in the human body have been listed. In snakes the pectoral 

 and pelvic girdles and rudimentary appendages are present in the 

 skeleton but do not appear externally. Not a single group of higher 

 animals lacks rudimentary structures that are found functional in 

 lower forms. 



Comparisons of Organs and Organ Systems: Chapters VI and 

 VII are very extensively devoted to a resume of the comparison of 

 the various organ systems and may be re-examined with profit, for 

 such comparisons bring to light clear evidence for evolution as 

 exhibited throughout the animal kingdom. 



Continuous and Divergent Series: The nature of the evidence 

 permits its division into two categories: continuous series and 

 DIVERGENT SERIES. An example of a continuous serial evolution is 

 shown by the vertebrate heart (Figs. 118-122). In the lower verte- 

 brates this structure is essentially a tube, inserted directly into the 

 blood system as a propelling organ. In the Amphibia, with the air- 

 breathing character appearing, there is a partial division of the 

 propelling organ into two separate systems, one concerned with 

 distributing the blood throughout the body and the other with its 

 aeration. The two are not completely separated, however, for the 

 heart of the amphibian has but a single ventricle in which it is 

 possible for some mingling of the blood of the two systems to 

 occur. In some of the Reptilia the separation of the blood streams is 

 not quite complete, the septum between the two ventricles being 

 perforated. But in other reptiles and in Aves and Mammalia that 

 portion of the heart which propels the blood to the lungs and that 

 portion which propels it throughout the body are completely sepa- 

 rated. The steps have been continuous from a single organ of 

 propulsion inserted directly into the system to two separate organs 

 in parallel, one concerned in the aeration of the blood and the 

 other in its general distribution after aeration. 



