1869.] DAWSON — IDEAS OF DERIVATION. 125 



convince many of his auditors that the gigantic and grotesque 

 Iguanodons of the Mesozoic rocks might have been the pro- 

 genitors, if not of wrens and titmice, at least of ostriches and 

 cormorants. Yet he could not have phiced together any two 

 members of the supposed series without convincing any naturalist 

 that an enormous gap had to be filled between them. Prof. 

 Owen, writing to naturalists, does not attempt this sort of 

 intellectual sleight of hand, but presents, as a case in point, the 

 supposed progenitors of the liorse. That useful quadruped was 

 preceded in the tertiary period (Miocene and Pliocene) by a 

 horse-like animal, the Hipparion, which, among other things, 

 differed from its modern representative in having its splint bones 

 represented by two side toes, a conformation supposed to adapt it 

 to locomotion on soft and swampy ground. The Hipparion was 

 preceded in the earlier tertiary (Eocene) by the Palasotherium, 

 in which the side toes were still further developed so as to touch 

 the ground, giving the foot a tridactyle character. These 

 relations induce Owen to believe that these forms may be an 

 actual genetic series, the species of Palgeotherium passing 

 through a succession of changes into the modern horse. Perhaps 

 this case, as put by Prof. Owen, affords as fair an example as we 

 can obtain of the bearing of a derivative hypothesis. The three 

 genera in question are closely allied. They succeed each other 

 regularly in Geological time. The horse shows in his splint 

 bones rudiments of organs, which, serving little apparent purpose 

 in him, were more fully developed and of manifest use in his 

 predecessors. Modern horses have occasionally shown a tendency 

 to develop the side toes, as if returning to the primitive type. 

 Taking this as a fair example of derivation, and admitting, I'or 

 the sake of argument, its probability, let us consider shortly some 

 of the questions that may be raised with regard to it. These are 

 principally two. 



1. What limits, if any, must necessarily be set to such an 

 hypothesis, and what relations does it bear to the origin of life ut 

 first and to the succession of animals in Geological time ? 



2. What causes may be supposed to have led to such deri- 

 vation ? 



Under the first head, we have to enquire as to the limits set to 

 derivation by the structure of organic beings themselves, and by 

 the physical conditions and changes which may affect them. J[t 

 will be convenient to consider these together. 



