HISTORICAL INTERPRETATION OF FORM 311 



A further instance of the historical interpretation of 

 animal structure, taken from quite a different field, is afforded 

 by the speculations of Dollo l on the ancestral history of the 

 Marsupials. In a brilliant paper of iSSo 2 Huxley made the 

 suggestion that the ancestors of Marsupials were arboreal 

 forms. " I think it probable," lie wrote, " from the character 

 of the pes, that the primitive forms, whence the existing 

 Marsupialia have been derived, were arboreal animals ; and 

 it is not difficult, I conceive, to see that, with such habits, it 

 may have been highly advantageous to an animal to get rid 

 of its young from the interior of its body at as early a period, 

 of development as possible, and to supply it with nourishment 

 during the later periods through the lacteal glands, rather 

 than through an imperfect form of placenta " (p. 655). Dollo 

 followed up this suggestion, which had in the meantime been 

 strengthened by Hill's discovery of a true allantoic placenta 

 in Perameles, by demonstrating in the foot of present-day 

 Marsupials certain features which could only be interpreted 

 as inherited from a time when the ancestors of Marsupials 

 were tree-living animals. These were the occurrence of an 

 opposable big toe (when this was present at all), the great 

 development of the fourth toe, the reduction and partial 

 syndactylism of the second and third toes, and in some cases 

 the regression of the nails. These characters were shown to 

 be typical of arboreal Vertebrates, and their occurrence in 

 forms not arboreal indicated that these were descended from 

 tree-living ancestors. Traces of an arboreal ancestry could 

 be demonstrated even in the marsupial mole Notoryctes. 



These are only two examples out of hundreds that might 

 be given. Present day structure was interpreted in the light 

 of past history; the common element in organic form was 

 seen to be due to common descent ; the existence of vestigial 

 and non-functional organs was no longer a riddle. 



There was even a tendency to concentrate attention upon 

 the historical side of structure, upon what the animal 

 passively inherited rather than upon what it personally 



1 "Les Ancetres des Marsupiaux etaient-ils arboricoles ? " Trav. 

 Stat. zool. Wimereux, vii., pp. 188-203, pis. xi.-xii., 1899. See also 

 Bensley, Trans. Linn. Soc. (2) ix., pp. 83-214, 1903. 



- Proc. Zool. Soc., pp. 649-62, 1880. Set. Mem., iv., pp. 457-72. 



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