262 BIOLOGICAL LECTURES. 



(2) A problem as to the modes in which evolution operates, 

 concerning which there has been much discussion, is whether 

 development is always by a series of direct and unswerving 

 changes, each successive step in a given phylum coming, in 

 every detail of structure, just so much nearer to the final 

 result. Some years ago I had occasion to make a careful com- 

 parison of several successive genera in the equine phylum, and 

 this study led me to the following conclusion. As a rule, 

 development is remarkably unswerving and direct, in a large 

 sense, yet in minor details a certain latitude is permitted, and 

 in these evolution may pursue a more or less zigzag course, 

 with many ups and downs. This conclusion is considerably 

 strengthened by what we have learned concerning the main 

 line of descent in the tylopodan phylum. Comparing the more 

 ancient genera of this series with its modern representatives, 

 we are at once struck by the remarkable difference in their 

 canine teeth. In Protylopus and Poebrotherium the canines are 

 very small and may almost be called incisiform, but in Gompho- 

 therium of the John Day the canines begin to enlarge, and 

 from that time onward these teeth become larger and larger, 

 until the formidable lacerating apparatus of the modern type is 

 attained. In the probable ancestors of the Uinta camel, Homa- 

 codon and Trigonolestes, the canines are relatively long and 

 pointed, but even though we should exclude those genera from 

 the series, the analogy of all the ungulate groups would justify 

 the assumption that the ancestors of Protylopus had canines 

 which were of fairly large size and formed effective weapons. 

 No one can imagine that in the Uinta genus these teeth are in 

 their primitive condition. We are forced to infer, then, that 

 the canines first dwindled to very small proportions, only to 

 enlarge again and become formidable. 



Another instance of much the same kind is afforded by the 

 history of the premolars. In Protylopus these teeth resemble 

 in general form those . of the other contemporary selenodont 

 genera and of the White River Leptomeryx, but they are dis- 

 tinguished by an incipient elongation in the antero-posterior 

 direction. In Poebrotherium the premolars are greatly elon- 

 gate, carrying much farther the process which was begun in the 



