Inheritance Explains Bio genetic Lcnv 219 



gelatinous envelope by compressing them between needles : 

 "If the needles were withdrawn again immediately after 

 the deformation, the embryo at once resumed its earlier 

 form. If on the contrary they were held in place for 

 several hours the deformation became from the first a 

 persistent one, and only after several hours would the 

 embryos resume their original form a proof that an in- 

 ternal adaptation to the new form had already com- 

 menced, but that this adaptation is nevertheless caused to 

 disappear again in the course of further development, per- 

 haps by the action of those very forces of growth which 

 bring about the restoration of the normal form, 

 and which were inhibited during the time of the 

 deformation." 166 



We have thought it worth while to mention again this 

 very characteristic example of the elasticity of develop- 

 ment, because it, better than others which we have 

 already mentioned in the course of our investigation of 

 the cause of this elasticity, helps us to explain the rule 

 inviolably followed in the evolution of species, of the 

 addition of new phylogenetic characters to those already 

 present. For from this it is very evident that those 

 phylogenetic characters whose appearance is caused dur- , 

 ing ontogeny to some extent by the action of external 

 influences, have the tendency to disappear again promptly 

 as soon as the cause which produced them has ceased to 

 act. So that, unless we have an extraordinary influence, 

 whose intensity and insistent action during ontogeny 

 through the course of successive generations give it an 



L88 Roux : Zur Orientierung iiber einige Probleme der embryonalen 

 Entwicklung. Zeitschr. f. Biol. ; Bd. XXI. Minchen. July 1885. 

 P- 515, 5i6. Gesamm. Abhandl. II. P. 245. 



