GENERAL ZOOLOGICAL FACTS AND THEORIES 295 



to the race and leading to extinction, structures such as paleon- 

 tologists tell us have appeared many times during the history of 

 the animal kingdom, and illustrated by the Irish stag whose 

 antlers became so enormous as to make it unfit for existence. 

 There appears to be a tendency for modifications to follow certain 

 definite paths, and for certain structures to progress in prede- 

 termined directions regardless of their utility, and without the 

 aid of natural selection. This evolution in a definite predeter- 

 mined direction is known as orthogenesis. 



There are a number of theories of orthogenesis. The one most 

 in favor is that which ascribes the control of modifications to the 

 direct effects of physicochemical factors on organisms. Some of 

 the arguments for orthogenesis are derived from the following 

 phenomena: the presence of similar variations in species belong- 

 ing to one family, the development of disadvantageous structures 

 leading to extinction (e.g. the antlers of the Irish stag), the limits 

 of variation due to the chemical composition of the body, the 

 influence of one organ upon another, which limits variation, and 

 the persistence of variations in definite directions so that evolu- 

 tion is not in radiating lines as natural selection demands (264). 

 According to orthogenesis, certain lines of development remain 

 stationary, while others advance. 



The theory of orthogenesis in some form or other is accepted 

 by many specialists, working in widely separated fields. Of these 

 may be mentioned Eimer (239, butterflies, birds, and lizards), 

 Whitman (280, pigeons), Tower (270, beetles), and Ruthven (266, 

 snakes). The papers of these men furnish excellent concrete 

 examples of how orthogenesis is supposed to operate. 



THE MUTATION THEORY. A mutant is an organism that dif- 

 fers from its parent in certain well-marked characteristics, which it 

 transmits to its offspring. De Vries is the foremost exponent of 

 the mutation theory, and it is his work that has given it world- wide 

 recognition. The conclusions of de Vries are based on a study of 

 the evening primrose ((Enotkera lamarckiana) . The mutation 

 theory may be stated in his words as follows: " The way in which 



