46 GENESIS OF THE ARIETIDiE. 



fishes and birds at an early stage exhibiting this peculiarity. Dr. C. S. Minot 1 

 has declared, upon similar grounds, that there was nothing in man's structure 

 which justifies us in considering him as morphologically a higher animal than the 

 more specialized Mammalia. Though not willing to indorse this statement as a 

 whole, it is in part true. 



Notwithstanding the increased functional power of the brain, the loss of cer- 

 tain characters which once marked the progress of ancestral structures remains 

 to be accounted for. In fact, as suggested by Professor Shaler, the body and its 

 organs exhibit the evidence of having belonged originally to a horizontal type, 

 and in accommodating itself to a vertical position has succeeded in adapting its 

 parts to meet new and more complex conditions, and a new position with relation 

 to gravity, without having made very essential changes in hereditary structures. 

 Man is a walking simian biped, or, as Cope describes him, a " pentadactyle plan- 

 tigrade bunodont." 2 Nevertheless man is also a highly specialized type. The 

 extraordinary development of the legs, the increased size of the big toe, the dif- 

 ferentiation of the feet, the broadening out of the chest and sigmoid curve of the 

 backbone, the differentiation and shortening of the arms, the changes in the pel- 

 vis, and other correlative specializations, have probably been introduced through 

 the exercise of the parts in an upright position, as was originally pointed out 

 by Lamarck. 3 



There is, of course, a vast gap to be bridged between man and Oxynoticcras 

 oxynotum, but they are none the less types belonging to the same general category 

 of geratologous forms. They have arisen suddenly, and present alike the com- 

 mingling of degenerative and specialized characters. Such geratologous species 

 show us that any form having a highly accelerated development, and producing 

 suddenly some new and unexpected characteristics, as, for example, the hollow 

 keel of Oxynoticeras, or the peculiarities of man's structure, may have, in asso- 

 ciation with these, many retrogressive characteristics. 



The extreme example of Baculites-like shells are very distinct from these, 

 and show us that the development of geratologous characteristics may some- 

 times take place without the introduction of new characteristics. In such cases, 

 the adults may resemble their own young more completely than in man, or in 

 other examples given above, and may be similar in aspect to the primitive radical 

 of the whole group, as is Baculites when compared with Orthoceras. Such con- 

 siderations and others given above have led us to compare this class of forms in 

 which degeneration is complete with the oldest stage of decline in the individual, 

 and we have accordingly placed them in the same category as nostologic forms. 

 This serves to distinguish them from partially degraded forms, which can be 

 called geratologous, or from forms that resemble the first senile stages of the 

 individual, and can be termed clinologic forms. 



In order to account for high degrees of specialization, and their tendency to 



1 Amer. Assoc. Adv. of Science, August, 1881. 



2 Origin of the Fittest, p. 266, referring of course to the retention in the organization of ancient ances- 

 tral characters. 



3 Philosophie Zoologique, I., Pt. I , chap viii , Quelq. Observ. rel. a 1'Homme. 



