GROUNDS AND CONSEQUENCES OF EVOLUTION. 341 



center in the remoter past. We expect to see the con- 

 secutive terms in these various series graduating structu- 

 rally into each other, and every characteristic conformed 

 and arranged as if there had been a gradual descent of 

 all our modern mammals, along a set of diverging lines 

 from some primitive, plantigrade, five-toed ancestor. 



This is the generalization which the known facts and 

 the known tenor of the facts authorize us to draw. But 

 when we shall have become convinced of the existence of 

 such a complete series of successional lines, we shall not 

 yet have the demonstration of a genealogical connection 

 between any two terms in any series. It will still be 

 supposable, as stated in a previous article, that each term 

 is a separate origination. We shall not yet have the 

 demonstration that one specific or generic type has ever 

 passed by modification, in the course of generations, into 

 another specific or generic type. We shall have no 

 demonstration that it is in the economy and plan of na- 

 ture to permit specific transmutations. We may fairly 

 argue that the facts accord with the theory of derivation, 

 and are best explained bv that theorv, and lend it a high 

 degree of probability ; but we should feel our confidence 

 materially strengthened if we could detect nature in the 

 act of effecting a transition from species to species. 



We have therefore, in the third place, the variational 

 evidence. This consists of a bodv of facts tending to show 

 that a species is not a primordial and permanent organic 

 form, but only the existing phase presented by a line of 

 progressive changes. Much light has been thrown upon 

 this subject within a few years. Some cases of transmu- 

 tation have been actually traced, and evidence has been 

 gained that the gradational series connecting species of 



