506 MR. ST. G. MIVART ON THE LEMURS. [May 20, 



Manifestly, then, very great caution is necessary in discriminating 

 between genetic characters and characters purely or mainly adaptive. 



Experience has more and more persuaded me that the number of 

 similar structures which have arisen independently is prodigious. 



The elaborate investigations of my friend Mr. Parker constantly 

 bring before us an increasing number of complex cross relations and 

 more and more entangled interdependencies ; and I am convinced 

 that by means of such careful and minute researches many of the 

 genealogical trees which have been developed with the rapidity of 

 the fabled " bean-stalk " are destined to enjoy an existence little less 

 ephemeral. 



The notion that "similarity of structure" necessarily implies 

 " genetic affinity " can no longer be ranked as a biological axiom. 



If, then, it is so difficult to decide as to which characters are ge- 

 netic and which adaptive, the second question can be answered at 

 once. Evidently anatomical science does not now enable us to 

 group even the Mammalia by genetic characters ; yet surely the 

 main features of Mammalian classification may be considered to be 

 satisfactorily established. 



The third question concerns the exclusion from any order of all 

 species which cannot be supposed to have sprung from an ancestor 

 common to them and to all the other species. 



To confine our attention to the Mammalia, can it be considered 

 certain that the Balcenoidea and the Delphinoidea sprang from an 

 ancestor which at the same time was the ancestor of no species be- 

 longing to any other special order of existing Mammals 1 And 

 if we could demonstrate that such had not been the case, would 

 that be a reason for breaking up the very natural and, on the whole, 

 homogeneous order Cetacea ? 



Again, can we feel any certainty that Orycteropus has descended 

 from the same stock as that whence the American Edentates de- 

 scended ? yet who would place it in a separate order 1 



Once more, it may well be that the Artiodactyla and Perisso- 

 dactyla are entirely independent genetically beyond the fact that 

 they are both Mammals ; yet no one can deny that the Ungulata 

 form a very natural group. 



As to the fourth question — whether, namely, no common de- 

 scendants should be classed in two different orders, — it seems 

 reasonable that convenience should determine our practice. If the 

 number of species of any one group is overwhelming, and if the 

 complicated subdivisions of its families, subfamilies, and genera are 

 very great, surely, then, convenience should determine us to subdivide 

 them into two or even more orders. 



Similarly as convenience may induce us to separate into distinct 

 ordinal groups, so convenience may reasonably induce us to unite in 

 one group forms which, whether descended from a common ancestor 

 or not, undeniably constitute a well-defined and convenient aggre- 

 gation. 



As has been said, it may be that the characters which unite the 

 Artiodactyla with the Perissodactyla are merely adaptive functional 



