376 V. THE NATURE OF EVOLUTION 



Such evolution along an orthogenetic course must have depended 

 upon the gradual change of a gene in a certain direction. If some 

 gene began to change its structure gradually, the organisms having 

 the gene would be compelled to change the character according to the 

 change of the gene. However, as seen also in ontogen3% the direction 

 of the change would be not always straight, although in the majority 

 of cases it appears to bave occurred in straight lines. 



This supposed property of genes to alter the structure gradually 

 must be of the utmost importance for the establishment of evolution. 

 Full discussions on this problem will be made next. 



2. Phage as a Free Gene 



The gradual alteration of genes must be reversible since the indi- 

 vidual development from the germ cell is given rise to by this change, 

 owing to the reversibility the developed structure can be reduced 

 again to the germ cell. On the other hand, genes may frequently 

 undergo a sudden, irreversible change. The change of organisms 

 due to such an abrupt, irreversible alteration in a gene is called 

 mutation. There seem, therefore, two types of genie changes, one of 

 which is this irreversible, abrupt change, the mutation, and the other a 

 gradual, reversible change. The nature of these two types of changes 

 will be considered below. 



Phage among many viruses can be regarded especially as a free 

 gene, for it contains a large amount of desoxyribonucleic acid, the 

 nucleic acid peculiar to genes. Here, it should be noted that phage 

 thus possessing a free gene nature will change gradually in a certain 

 direction under the influence of formalin as shown in Fig. 35. If 

 phage is regarded as a gene, formalin should be compared to an en- 

 vironmental factor having an influence upon the gene. In so far as 

 the factor is present the change appears to proceed in a certain 

 direction. 



The evolution along an orthogenetic course may thus be attributed 

 to the existence of some environmental factor or factors which, like 

 formalin, caused the gradual change of a gene in a certain direction. 

 Orthogenesis, therefore, would come to a stoppage, if the environmen- 

 tal factor in question was removed. Thus it may be concluded that 

 for the establishment of orthogenesis continued existence of a certain 

 stimulus or an environmental factor which caused the change of a 

 gene would always be needed. 



The proteins of our somatic cells are bound to undergo senes- 

 cence since the time of birth, a phenomenon which may be due to the 



