APPARITION OF SPECIES in 



actual genetic series, the species of Palceotlierium or 

 Eohippus passing through a succession of changes 

 into the modern horse. Perhaps this case affords as 

 fair an example as \ve 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 ap 

 parent 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, for the sake of argument, its probability, 

 let us consider shortly some of the questions that 

 may be raised with regard to it. These are princi 

 pally 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 at first, and to the succession of 

 animals in geological time ? 



2. What causes may be supposed to have led to 

 such derivation ? 



Under the first head we have to inquire 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. It will be 

 convenient to consider these together. 



Supposing that Palceotlierimn, Hipparion, and 



