286 THE POPULAR SCIENCE MONTHLY. 



I pointed out, in fact, that, as we go back in time, the great inter- 

 vals which at present separate some of the larger divisions of ani- 

 mals become more or less completely obliterated by the appearance 

 of intermediate forms, so that if we take the particular case of reptiles 

 and birds, upon which I dwelt at length, we find in the mesozoic rocks 

 animals which, if ranged in series, would so completely bridge over 

 the interval between the reptile and the bird that it would be very 

 hard to say where the reptile ends and where the bird begins. Evi- 

 dence so distinctly favorable to evolution as this is far weightier than 

 that upon which men undertake to say that they believe many im- 

 portant propositions ; but it is not the highest kind of evidence at- 

 tainable for this reason, that, as it happens, the intermediate forms 

 to which I have referred do not occur in the exact order in which 

 they ought to occur, if they really had formed steps in the progression 

 from the reptile to the bird ; that is to say, we find these forms in 

 contemporaneous deposits, whereas the requirements of the demon- 

 strative evidence of evolution demand that we should find the series 

 of gradations between one group of animals and another in such order 

 as they must have followed if they had constituted a succession of 

 stages, in time, of the development of the form at which they ulti- 

 mately arrive. In other words, the complete evidence of the evolu- 

 tion of the bird from the reptile what I call the demonstrative 

 evidence, because it is the highest form of this class of evidence ; that 

 evidence should be of this character, that in some ancient formation 

 reptiles alone should be found ; in some later formations birds should 

 first be met with ; and in the intermediate strata we should discover 

 in regular succession those forms which I pointed out to you which 

 are intermediate between reptiles and birds. 



The proof of evolution cannot be complete until we have obtained 

 evidence of this character, and that evidence has of late years been 

 forthcoming in considerable and continually increasing quantity. 

 Indeed, it is somewhat surprising how large is the quantity of that 

 evidence, and how satisfactory is its nature, if we consider that our 

 obtaining such evidence depends upon the occurrence in a particular 

 locality of an undisturbed series deposited through a long period of 

 time, which requires the further condition* that each of these deposits 

 should be such that the animal remains imbedded in them are not 

 much disturbed, and are imbedded in a state of great preservation. 

 Evidence of this kind, as I have said, has of late years been accumu- 

 lating largely, and in respect to many divisions of the animal king- 

 dom. But I will select for my present purpose only one particular 

 case, which is more adapted to the object I have in view, as it relates 

 to the origin, to what we may call the pedigree, of one of our most 

 familiar domestic animals the horse. But I may say that in speak- 

 ing of the origin of the horse I shall use that term in a general sense 

 as equivalent to the technical term HJquus, and meaning not what you 



