ARRESTS OF DEVELOPMENT 125 



sions " on the flimsiest of evidence. The word reminds one of the 

 placard beside a " free toom," " Rubbish Shot Here." It will be 

 no small gain if we can define the conception into one of greater 

 utility by criticism and elimination of alleged instances. In 

 this criticism we have especially to bear in mind that the term 

 " reversion " is not merely descriptive of the direction which varia- 

 tions may take ; it implies that this direction — ancestor- wards — is 

 hereditarily determined, that it is due to the reassertion of latent 

 ancestral characters. 



After we have sifted out those phenomena whose inclusion 

 under the rubric " reversion " is illegitimate, we shall see more 

 clearly what may be plausibly interpreted as due to the re- 

 emergence of ancestral characters after a more or less prolonged 

 period of latency. In regard to the latter it will appear, how- 

 ever, that, although the reversion hypothesis is applicable, other 

 interpretations are not necessarily excluded. Finally, it will be 

 seen that true reversions may arise {a) in a pure-bred race, 

 and {b) much more frequently as the result of hybridisation. 



Arrests of DeYelopment. — Though popular travesties have 

 reduced a luminous idea to an absurdity, it remains in a general 

 way true that the individual development, especially in the 

 stages of organ- forming, is in some measure a recapitulation of 

 the racial history. Although it is more picturesque than accurate 

 to speak of " every animal climbing up its own genealogical 

 tree," there is a suggestive general resemblance between the 

 stages in the individual development of organs, such as heart, 

 brain, and kidneys, and the stages in the supposed racial evolu- 

 tion of the same. 



Now, it not infrequently happens that the recapitulation 

 is notably incomplete, that the development of an organ stops 

 before the normal " finished form " has been attained. 



Through defective nutrition or other untoward conditions of 

 nurture, the expression of the inheritance is inhibited. The 

 organism is not able to perfect itself in all its parts; not, we 



