78 WARFARE IN THE HUMAN BODY 



As yet, little is known of myogenesis. Like a neuron, a 

 muscle cell seems to last a lifetime, and though both may 

 degenererate or die, neither proliferates after the early 

 period of development. But whatever their histogenesis, 

 new fibres do appear in evolution. Harvey did not refuse 

 to believe in the validity of his own conclusions, because 

 he lived before Leeuwenhoek. With considerable hesita- 

 tion I venture to suggest that morphogenetic stress is at 

 its height during foetal development. The child in utero 

 has not, perhaps, the calm and happy life commonly 

 attributed to it. On the contrary, it probably leads a 

 strenuous existence, and if it inherits a new weakness this 

 is shown just where and when new stresses find plastic 

 embryonic tissues to respond to them. If such a specu- 

 lation is sound it accounts for many phenomena. But in 

 any case, whatever the machinery of inheritance and 

 evolutionary repair, it is certain that new fibres arise where 

 they are needed. 1 The origin of the cremaster muscle as a 

 lately evolved support for the testis certainly strengthens 

 this view. Hunter could not account for its appearance 

 during embryonic life, when the testis occupies its original 

 position, and the cremaster serves no purpose. In the 

 testiconda such a muscle is not found. It must obviously 

 have arisen as the result of stress during the evolutionary 

 descent of the testis, and cannot be accounted for except 

 by such stresses and fcetal hormonic influences. 



If such views in any way represent the biological 

 history of the heart, it is obvious that many of the opinions 

 of variation usually held are without foundation. Every 

 variation is definitely caused ; it is in no sense accidental 

 or spontaneous ; it may not even be at once advantageous 

 to the individual ; on the contrary, it may be a severe 



1 See Appendix B. The Peroneus Tertius. 



