EVOLUTIONAL ZOOLOGY 



GEORGE LEFEVRE 

 Professor of Zoology 



DEFINITION OF ORGANIC EVOLUTION 



From the time of the early Greek philosophers to the 

 present, natural objects and events have been regarded from 

 two opposing points of view which are the outcome of the 

 contemplation of two apparently contradictory aspects of na- 

 ture, namely, its seeming stability and its changefulness. 

 According as men have been appealed to by one or the other of 

 these aspects, they have held either a static or a genetic view. 

 On the one hand, the apparent fixity of nature and the con- 

 tinual recurrence of the same specific forms in definite cycles 

 have impressed themselves upon the minds of those whose 

 interest in nature is restricted to a description of forms and 

 the abstraction of types from these forms as the basis of con- 

 ceiving ideal relationships. 



In contrast with this conception, another school of natu- 

 ralists, impressed by the restless change and movement of 

 everything, have been led to look beyond the immediate ap- 

 pearances of nature in an attempt to discover causal relation- 

 ships and orderly sequences, and to penetrate the mystery of 

 the genesis, the coming into being, of those specific forms 

 which observation reveals to us. From speculations of this 

 kind concerning the origin and development of natural phe- 

 nomena have arisen genetic views which, not content with 

 a purely descriptive picture of nature as a whole, have sought 

 to answer the question how have these things come to be 

 what they are? what is their history? 



As in astronomy and geology, so in biology, these two 

 conceptions of nature have been conspicuous throughout the 



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