66 THE PRINCIPLES OF HEREDITY 



adult organism. The animal in its development seems to 

 retrace some of the steps it has already made. It is clear 

 the retracement was made in the life-history as well. The 

 horse presents us with another striking example. The 

 remote ancestor of the modern horse, the Hipparion, had 

 three functional toes. The embryo of the horse has also 

 three toes of considerable size. But the two outer toes 

 in each limb, which in the embryo are nearly as well 

 developed as in the Hipparion, degenerate partially during 

 development, so that the horse is born with only one func- 

 tional toe. " Occasionally a foal is born with two hoofs on 

 one or more of its limbs ; at very long intervals a foal appears 

 with three hoofs on one or more of its limbs." Now when 

 the outer toes persist in the individual we have plainly, in a 

 real sense, an arrest of development. The toes remain in 

 the embryonic, the ancestral condition ; the whole life-history 

 is not recapitulated. But in the normal horse there is no 

 arrest of development ; the recapitulation is carried out to 

 its full extent, though, in the later stages, in a reversed 

 direction. 



110. As in the case of other forms of Natural Selection, 

 the principal, if not the sole, effect of reversed selection, 

 therefore, appears to be a prolongation, not an abbreviation 

 of the life-history. It causes in the more mature individual 

 a deterioration of organs which have become injurious to it, 

 but which, during a former period of the life-history, were 

 useful, or which are still useful, or at any rate comparatively 

 harmless, to the less mature individual. When, therefore, 

 we see a structure or an organ better developed in the 

 embryo than in the adult, in the larva than in the imago, 

 we must conclude that reversed selection has been at work. 



