EVOLUTIONARY THEORIES SINCE DARWIN 243 



that the earliest beginnings of any variation must 

 be too sHght to be useful, or as the term went, to 

 have selective value. 



It has been noticed by a number of naturalists that 

 certain animals seem to carry the development of a 

 peculiarity altogether too far. It is seen for instance 

 that in the Irish Elk, which has for some time been 

 extinct, the horns were so enormous as to be a source 

 of danger rather than of assistance to their owner. 

 It was said that the tendency to produce heavy horns 

 had gained, as it were, a sort of momentum, and that 

 this impulse had carried the development beyond a 

 safe limit. The Irish Elk became extinct because his 

 horns were too heavy. During the Mesozoic period 

 the reptiles grew too large. They seemed to have 

 carried size to a point at which it became a danger 

 instead of a help. They completely passed out of 

 existence, leaving behind them only very much 

 smaller reptiles. 



Eimer, of Germany, has based on facts like these 

 his theory of Orthogenesis. He says that variations 

 in animals are not indefinite and in every direction, 

 but that they follow along clear and definite lines. 

 These lines, in the case of the elk and of the Meso- 

 zoic reptiles, developed too far, but ordinarily the 

 effect of such a tendency is distinctly lieneficial to 

 the animal. It particularly assists in carrying on for 



