CHAPTER V 



GRADUAL ALTERATION OF GENE AND 

 ITS RELATION TO MUTATION 



1 . Orthogenesis 



If the miniature copy of phylogeny is seen in ontogeny, phylogeny 

 must have been raised by the same continuous changes as ontogeny. 

 In ontogeny the destination is settled preliminarily, the destination 

 bound to reach is the original, advanced structure, and since the deve- 

 lopment is only the recovery of the original structure by means of 

 the reversibility, it can occur rapidly, whereas in phylogeny as the 

 destination is unsettled and a new course has to be made, its progress 

 must be very tedious. 



Nevertheless, the change of genes in phylogeny, like in ontogeny, 

 would occur under the influence of environmental factors, wherein 

 hormones would play an important part as will be considered later. 

 This change of genes, as shown in ontogeny, would proceed in a 

 certain direction and therefore organic evolution would occur gradually 

 along a certain course. In fact, it is stated that evolution has occurred 

 in straight lines or definite directions as indicated by the term 

 "orthogenesis." 



The term "orthogenesis" has been used to indicate a straight 

 genesis or evolution directed by internal factors. It is claimed that 

 lines of evolution were not haphazard but were determined, not by 

 natural selection but by law of organic growth. True, in some cases 

 evolution seems to have proceeded along a certain course. For example, 

 the increasing size of such forms as the horses, elephants, and dino- 

 saurs, the reduction of digits in the horse series. Many fossil series 

 suggest that types begin simply and evolve into complexities, such as 

 overgrown spines and tusks, the overdevelopment being followed by 

 extinction. Some zoologists and a larger number of paleontologists 

 claim that evolution of this general sort has occurred so often in 

 widely different types of animals that it can be explained only as a 

 result of fundamental forces inherent in a race and carrying it along 

 a straight or orthogenetic course. 



