216 THE PROBLEMS OF EVOLUTION 



pouches as a feature of his embryological develop- 

 ment, although he has no such need for them. Dur- 

 ing the ages of his past evolution each step was 

 accomplished in association with these pouches 

 and their derivatives, hence they persist in the 

 development of structures which are now essen- 

 tial. It is unnecessary to write at length of the 

 importance of the endocrine glands developed in 

 this part of the body. They may originally have 

 been incidental, but they existed and they are now 

 indispensable, along with the pharyngeal pouches 

 from which they develop. 



These points are strictly in harmony with the 

 facts of evolution as disclosed by palaeontology 

 in long phylogenetic series, and with the limited 

 experiments which have demonstrated an appar- 

 ent inheritance of acquired characters. In the 

 former case we find gradual change under a grad- 

 ually changing environment to be a common condi- 

 tion. In the latter we find the gradual develop- 

 ment of a character and its gradual disappearance 

 in succeeding generations convincingly demon- 

 strated, especially in Woltereck's daphnids. 



Gradual appearance and gradual disappearance, 

 however, introduce the problem of reversibility. 

 If the changes under discussion are evolutionary, 

 one may well ask how they harmonize with the 

 generally accepted principle of irreversibility. It 

 seems, first of all, that this principle is based upon 



