GREGORY: NOTHARCTVS, AN AMERICAN EfX'ENE PRIMATE 



193 



(4) Another function of the Linnsean system which has been greatly emphasized by some modern 

 authors is that of indicating direct ancestral relationship of earlier to later forms which are placed by 

 these authors in the same family, even though they may represent widely different stages of evolution. 

 The result is that several very ancient genera which may be closely allied through the possession of many 

 important common characters will be widely separated in such systems, being placed in the different 

 modern families to which they are supposed to have given rise. 



Naturally, the application of the Linnsean system in practice is beset by many difficulties arising 

 either from the poverty of the available material or from its relative abundance. In both cases the 

 chief obstacle to the recognition of natural groups and of true evolutionary phyla is the facility with which 

 nature produces similar adaptations in related, and only to a somewhat less degree in widely removed, 

 phyla. 



The general, though tardy, recognition of the deceptive effects of analogous evolution has inspirtnl 

 such a degree of caution in many paleontologists that some refuse to admit the relationship or consanguin- 

 ity of later to earlier forms unless a practically complete series of intermediate forms be known. In the 

 lack of such complete series those who have an almost unlimited faith in the deceptive powers of analogous 

 evolution confess their own inability to distinguish analogous from homologous characters; they will, 

 therefore, probably read with considerable scepticism the opinion of the present writer that the ability 

 to distinguish analogous from homologous characters grows with practice and is conditioned, first, by 

 the extent of the investigator's experience in recognizing such differences among other orders of verte- 

 brates, and, secondly, by the thoroughness of the investigator's knowledge of the divergent trends of 

 evolution in the particular group under examination.^ 



After the descendants of a common stock diverge from it they frequently evolve many parallel, 

 independently acquired, but in a sense homologous characters, and such resemblances in independently 

 acquired homologous characters are usually more numerous between nearly allied stocks than between 

 those which diverged at a very remote epoch. So that "analogous evolution" does not always tend to 

 obliterate ancestral relationships and bring about false associations. 



Reasons for Assigning Adapinic and NotharctincC to the Same Family 



Fully recognizing the value and interest of Dr. Stehlin's conclusions, the weight of his authority, 

 and the likelihood that his finely divided "families" will be approved and adopted by other investigators 

 who favor analytical rather than synthetic classifications, the present writer will cheerfully abandon his 

 own classification of the Lemuroidea whenever, and as soon as, it shall be shown to be erroneous. After 

 prolonged and repeated consideration, however, it has seemed that the groups therein recognized are to 

 a greater or less extent "natural" groups and that they are reasonably free from intrusive or convergent 

 admixtures. The classification is also in historic continuity with the main line of anatomical and sys- 

 tematic research on the Lemuroidea and the characters selected as diagnostic have been, so far as possible. 



^ If the writer fails in the problem in hand to distinguish between analogous and homologous characters it will be in indication of 

 his own incompetence rather than of lack of opportunity for acquiring the necessary experience. In the Department of Vertebrate 

 Palaeontology in this Museum the phenomena of convergence and divergence have been studied by a number of investigators for many 

 years past, with the generally growing conviction that in most cases the deceptive effects of analogous evolution may be recognized by 

 sufficiently thorough and comprehensive search for the divergent trends of evolution, and by a constant endeavor to discover the adap- 

 tive "purpose," as it were, of the observed changes in structure. 



2 Bull. Geol. Soc. Amer., XXVI, pp. 432-438. 



