SECOND OUTLINE OF Tltl'IT I'.KIM ULY 77 



found playing similar variations upon the primitive tritubercular type. 

 There are surprisingly few distinct types, but an almost unlimited number 

 of sub-types, or variations of form. As we descend among the older rocks 

 and the various series begin to converge, it becomes increasingly difficult 

 to distinguish the different orders by their teeth alone. Thus it came 

 about that all the Eocene monkeys were at first referred to the ungulates, 

 or to transition groups, as expressed in M. Filhol's composite term Pacii u- 

 Umuriens. 



Tritubercular Homologie*. 



Embryological Evidence. The progress which has been made in the 

 embryology of the teeth is largely in the matter of the succession of 

 double series, as indicated by vestiges of earlier and later sets of teeth, 

 the so-called milk and permanent sets. Embryogenesis, however, has also 

 led to a very minute study of the order of succession of the cones of the 

 molar teeth, and without entering into the matter in detail, it may be 

 briefly stated that all authors are unanimous in describing the cones of 

 the lower molar teeth in different groups as developing in the same order 

 in which they are supposed to have arisen in the past, according to the 

 tritubercular theory, namely : Protoconid, Paraconid, Metaconid, Hypo- 

 conid. In the upper teeth, on the other hand, embryogenesis has been 

 found to contradict the conclusions reached by the tritubercular theory of 

 palingenesis, for all authors have agreed that the order is Paracone, Metacone, 

 Protocone, instead of Protocone, Paracone, Metacone. When these facts were 

 first brought out by Taeker, Eose and others, the writer, with undiminished 

 confidence in the force of paheontological evidence, advanced as an explana- 

 tion the fact that the protocone had become secondarily reduced in the 

 upper molars, and that the embryogeny no longer recapitulated the order 

 of evolution. This explanation has received a measure of support in the 

 latest researches by Woodward [M. F.], in which it is shown that in those 

 Insectivora in which the protocone is still the most prominent cusp of the 

 x/il>cn'or molars, this cusp also appears first in embryogeny, the paracone and 

 metacone following. Woodward points out that this is not the case in 

 other Insectivora, for they agree with the Primates, Ungulates and other 

 types which have been carefully investigated, in the late appearance of 

 the protocone. Woodward infers from these conflicting facts that there 

 were two modes of cusp evolution within the order Insectivora, one in 

 which the protocone appeared first, and another in which the protocone 

 appeared third or last. Such a double genesis seems to the writer highly 

 improbable.* 



It is, however, certainly important, as Woodward and many others 

 have observed, to strengthen the palasontological evidence for the trituber- 



*[See, however, the opposing views on pages 123- 126, 227.] 



