X. THE MECHANISM OF EVOLUTION 411 



As was already discussed in detail, germ cells in general, whether 

 they are of animals or vegetables, may develop various characters in 

 response to the environments to which they are exposed. The seed of 

 the winter rye is to be ahvays exposed to low temperatures during 

 germination and so if in some individuals a character favourable for 

 the subsequent development was produced by this stimulus, the indivi- 

 duals capable of acquiring such a character under the effect of low 

 temperatures would be selected as the fittest, and as a result the in- 

 dividuals thus selected would fail to show usual development Avithout 

 preliminary exposure to the low temperature. 



4. The Degeneration of Organs and Atavism 



Organic evolution is induced by the structural evolution of the 

 genes, whereas the structural change of the genes is reversible. There- 

 fore, organic evolution itself must likewise be reversible. 



Our eyes are no doubt one of the most advanced organs. The 

 development of the eyes is probably mainly brought about by continuous 

 and exceeding uses, so that the cessation of the use will lead to the 

 return to the original, undeveloped state, that is, the eyes will become 

 atrophied by disuse. Thus, many species of fishes living in deep seas 

 or in caves are known to be blind. Horses kept in mines and man kept 

 in dungeons are said to have had their eyes so impaired that they could 

 be restored to sight only by gradual exposure to light, showing the 

 wonderful swiftness by which the eyes would degenei"ate. This sug- 

 gests that their evolution has occurred rapidly and comparatively 

 recently. 



The theory of orthogenesis holds true in so far as the environmental 

 effects including the stimulus from use are constant and unchanged. 

 Any change in the effects, of course, would lead to a change in the 

 direction of the evolution. In an extreme case, the direction may be- 

 come entirely reverse. For example, when human ancestors might live 

 on trees like apes, their feet might have been developed so as to be 

 able to grasp objects, but their leaving from the trees followed by the 

 cessation of the use of the feet to grasp, might result in the return to 

 the original form. Again, whales might be able to walk when they 

 lived on land, but their return to water was followed by the degenera- 

 tion of the legs. 



The reversibility of genes is attributed to the memory of the original 

 structure. Hence, the diminution in the memory will lead to the dif- 

 ficulty in the manifestation of the reversibility. Since newly acquired 

 characters may have strong memories, their disappearance may occur 



