194 EVOLUTION AND DOGMA. 



general, aside from any of the numerous theories 

 which have been advanced to account for the com- 

 monly accepted fact of Evolution. But, before 

 closing this protracted discussion, it is important, for 

 a proper understanding of our subject, to make a 

 few brief observations respecting the factors which 

 have been operative in the origination and develop- 

 ment of species, and to say a few words regarding 

 some of the most popular theories concerning the 

 modus operandi of Evolution. 



As has incidentally been observed in the forego- 

 ing pages, the principal factors of Evolution are: i, 

 the physical environment ; 2, the use or disuse of 

 organs ; 3, natural selection. The first two of these 

 were recognized by Lamarck ; ' while the third owes 

 its prominence to the labors and speculations of 

 Charles Darwin. In addition to these three factors, 

 two others have attracted some attention, namely, 

 sexual selection, suggested by Darwin, and physio- 

 logical selection, which was especially insisted on by 

 the late Professor Romanes. 



By physical environment are understood, among 

 other things, the external conditions of life, such as 

 temperature, nature of the soil, humidity, dryness 

 and rarity of the atmosphere. That organisms, 

 whether animal or vegetable, are markedly affected 

 by changes of environment has long been admitted, 

 and it suffices here to refer to the well-known results 



1 The action of the environment was not unknown to 

 Buffon, and hence some of his admirers are wont to speak of 

 this factor as " Buffon's factor." It was, however, reserved for 

 Lamarck to demonstrate the important role which environment 

 plays in causing variation of organic forms. 



