PALAEONTOLOGY AS A DISCIPLINE. 



53 



series so full, so complete, that no observer can hesitate to 

 accept them as representing actually or very nearly the succes- 

 sive steps of evolutionary change in the order in which they 

 occurred. Little confidence may, perhaps, be placed in these 

 phyla by those who have not made a special study of them, 

 and it may be imagined that fuller knowledge will require 

 them to be completely changed. But when we find such a 

 series as that of the horses, leading back by almost imper- 

 ceptible gradations from the great monodactyl living forms to 

 their little five-toed progenitors in the far distant Eocene 

 times, doubt becomes well-nigh impossible. A limit of error 

 is placed by the stratigraphical order, the geological and 

 morphological successions coinciding beautifully. Whatever 

 changes in the details of such a series, a radical reconstruction 

 of it is not in the least likely to be called for. Few observers, 

 if any, would now uphold the arrangement of the equine 

 phylum proposed by Kowalevsky, namely, Palczotherium, 

 AncJiitherium, Hipparion, Equus ; and yet it is surprising 

 to see how the general character of this series and the deduc- 

 tions as to the manner of evolution which may be drawn from 

 it agree with those made on the basis of the equine series as 

 we now have it. Kowalevsky's mistake merely consisted in 

 putting certain members of the side branches into the main 

 line of descent, and that similar errors have been made in 

 accepted phylogenies is not at all unlikely. The correction of 

 such errors will, however, change the general result but little, 

 and we may appeal with considerable confidence to the con- 

 clusion which legitimately follows from a study of these 

 phylogenies. 



Fortunately, the well-defined phyletic series which have 

 already been made out occur in very widely separated animal 

 groups, mammals, reptiles, cephalopods, brachiopods, echino- 

 derms, etc., so that the points in which they agree are apt to 

 prove of general application and validity. The cephalopods 

 are particularly valuable in this connection, because in them 

 the embryonic and young stages of the shell are preserved in 

 the adult, and thus conclusions have a distinct support from 

 embryological considerations. To recur to the linguistic 



