116 HEREDITARY CHARACTERS 



recapitulation, than to attempt to explain it in any other 

 way. 



" Progression implies a divergence from the ancestral 

 type, the complete recapitulation plus an addition ; whereas 

 regression always implies reversion towards the ancestral 

 type and incomplete recapitulation." 1 



Most of the cases of regression in domesticated animals 

 and plants are comparatively simple. The parent form is 

 still with us, and we are able to make a direct comparison. 

 The various breeds of domesticated pigeons have been 

 derived from the blue-rock, and we know that by stringent 

 artificial selection we are able to modify the characters of 

 the parent form to a very considerable extent in a few 

 generations. The recapitulation then, in the individual that 

 returns to the characters of the blue-rock, is very nearly 

 a complete recapitulation, and only a few recently added 

 characters are omitted. But sometimes the reversion to 

 older and the disappearance of later characters must be due 

 to some further cause than a failure to recapitulate com- 

 pletely. To return to the modern horse, the embryo re- 

 capitulates the development of the three toes of its remote 



ancestor, but as development goes on two of these toes 



, 



disappear. Here we have an addition to the recapitulation, 

 not a failure to recapitulate completely, involving the dis- 

 appearance of an already established character. This is 

 " reversed selection," and it is necessary to distinguish such 

 cases from simple regression. The three toes of the Hip- 

 parion must have been produced by natural selection. The 

 continued action of natural selection upon the descendants 

 eliminated the two outer toes in the adult animal, but this 

 was arrived at without the elimination of the stage in the 

 embryo where the three toes existed. When a foal is born 

 with two or three toes the reversion is a failure to recapitu- 

 late the later stages which have been produced under the 

 influence of natural selection acting in a reversed direction, 

 but the disappearance of the two outer toes in the adult 



1 Archdall Reid, op. cit., p. 61. 



