Epilogue: What Comes of Studying Vertebrates 797 



may be favorable, but beyond an optimum limit the large animal 

 becomes clumsy, unwieldy, and at a disadvantage in competition with 

 smaller and more agile animals. In all the vertebrate Classes except the 

 Cyclostomata, there have been groups whose members distinguished 

 themselves from others of the Class by attainment of relatively great 

 size. Some ancient amphibians were very much larger than the largest 

 existing amphibian, the 5-foot Japanese salamander. Such gigantic 

 animals as some of the dinosaurs and the South American ground 

 sloths became extinct long ages ago. In more recent times, contempo- 

 rary with early man, the super-elephantine mammoth and mastodon 

 and the largest known birds, the moa of New Zealand and the elephant- 

 bird (Aepyornis) of Madagascar, came to their end. Exceptional are 

 the whales. So far as is known, cetaceans now living include the largest 

 animals that ever existed. Whales are a very ancient race. Their 

 persistent endurance long after the giants of all other vertebrate 

 Classes have passed out is doubtless due to their aquatic mode of life. 

 Ocean-going whales can operate successfully at a vastly larger size 

 than would be practicable for a land animal. For analogous mechanical 

 reasons, it is not practicable to build an automobile as large as the 

 motor ship Queen Mary. 



Specialization Among Multiple Organs 



Evolution within a certain group of animals is necessarily the sum- 

 mation of consistently correlated adaptive changes in the constituent 

 organs of the individuals in the group. Some organs of an animal exist 

 in multiple; e.g., vertebrae, teeth, aortic arches, integumentary glands, 

 glandular derivatives of the pharynx, etc. In the evolution of such 

 multiple organs, some noteworthy peculiarities are to be seen. 



In a group of animals, adaptive diversification is accompanied by 

 increase in number of individual animals and in the extent of their dis- 

 tribution. In the evolution of a group or series of multiple organs, 

 primitively all alike, more or less diversification occurs, but it is com- 

 monly associated with decrease in number of organs and restriction in 

 the area of their distribution. The decrease in number of organs is cor- 

 related with increase in the size of the individual organ. For example, 

 in the more primitive fishes teeth are numerous, small, all alike, and 

 widely distributed over the oral surfaces. In most mammals they are 

 relatively few and large, differentiated into incisors, canines, premolars, 

 and molars, and restricted to the jaws. And again, in fishes numerous 

 small mucous glands are distributed over all the oral surfaces. In 

 mammals many small scattered oral glands persist, but the secretory 

 functions of the oral surfaces are mainly assumed by a few pairs of 



