GREGORY: XOrilARCTUS, AN AMERICAN EOCENE PRIMATE 



213 



Perhaps no two investigators would give exactly the same evaluation to the totahty of these resem- 

 blances and differences between the Adapinse and the Lemuridse. Dr. Stehlin, as above stated, beUeves 

 that the Adapinae are probably excluded from further evolution in the direction of the Lemuridae by the 

 five chief characters discussed above (pp. 211, 212). He therefore dissents from the conclusions of Leche, 

 Forsyth Major, and Schlosser that the Adapinse are more or less directly ancestral to the Lemuridse both 

 in dental and cranial characters. 



The writer, on the other hand, would not only endorse the general conclusion of the above-named 

 authors but would feel that nearly all the marked differences between Adapinse and Lemuridse are such 

 as might be expected to distinguish Eocene from modern Lemurs. The same characters of the dentition 

 which exclude the Adapinse from close relationship with the Indrisidse, Chiromyidse, etc., tend to unite 

 them with the Lemuridse. The resemblances in the construction of the brain-case, of the whole auditory 

 region, between Adapinse and Lemuridse are of the most intimate and fundamental character. The 

 differences all illustrate the general principle that any modern forms have attained their present status 

 through the following commingled and overlapping processes; (1) hypertrophy or progressive emphasis, 

 (2) retrogression and secondary simplification, involving coalescence and convergence or dedifferentiation 

 of adjacent parts, (3) change of trend in certain parts. 



Certain features of the dentition suggest that the Lemurinse have close relations with the Adapi» 

 parisiensis group while the C'hirogaleinae may be related to the Adapts (Leptadapis) magnus group. At 

 any rate the remote ancestors of the Lemuridse should be closely related to Adapts sciureus. 



From all this it will be apparent that the writer gives no assent to the general principle that it is 

 impossible to discover the relationships of modern to ancient families until all the missing stages from 

 successive formations shall be discovered. The writer believes on the contrary that a sufficiently thorough 

 and comprehensive morphological and systematic analysis of existing material will often yield evidence 

 of great and permanent value. Morphological analysis alone, in addition to the valuable though imper- 

 fect palseontological evidence, is quite sufficient to establish beyond reasonable doubt that the Cetacea 

 have been derived from terrestrial Placental mammals, that the Hominidse represent a terrestrial offshoot 

 of an arboreal catarrhine stock, that the various members of the Lemuridse are more nearly related to 

 each other than any of them are to the Indrisidse, etc. The more precise questions may, indeed, have to 

 await further discovery, but the limits to our knowledge of phyletic relations must be determined not 

 by any such a priori negative principle as Dr. Deperet ^ and Dr. Stehlin have sought to establish, but by 

 intensive, prolonged and comprehensive investigation of each case. 



The broader stages in the origin and evolution of the Lemuridse may be summarized as a working 

 hypothesis as follows: 



Stage 1. — Paleocene protolemurines (hypothetical). Insectivorous-frugivorous. if C'l P| Alf. 

 Lower incisors small, suberect; lower canine subcaniniform. Premolars simple: upper molars tritu- 

 bercular, with cingulum-hypocone, proto- and metaconules; lower molars tuberculosectorial. Orbits 

 relatively small. Lacrymal within orbit. Brain-case small. Moderate muscle crests. Hypotympanic 

 sinus not covering tympanic ring. Brain macrosmatic, with small frontal lobes and smooth cerebra. 



Stage 2. — Eocene Adapinse. Insectivorous-frugivorous-omnivorous. if Ci P| Mf. Lower 

 incisors and canines as in Adapts sciureus. P^ with incipient metacone; upper molars with low cingulum- 

 hypocone and sharp protoloph. Lower molars losing paraconid, acquiring metacristid. Lacrymal 

 within orbit. Brain-case widening. Jaw stout. Muscle crests very high, zygomata powerful. Hypo- 



> Les Transformations du Monde Animal. 



