Inheritance Explains Biogenetic Law 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“If the needles were withdrawn again immediately after 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 
466Roux: Zur Orientierung tiber einige Probleme der embryonalen 
Entwicklung. Zeitschr. f. Biol.; Bd. XXI. Mlinchen. July 1885. 
P, 515, 516. Gesamm. Abhandl. II. P. 245. 
