CLASSIFICATORY CHARACTERS 



upon as ancestral to both. The method of Palaeon- 

 tology, however, has at present to be supplemented by 

 comparative anatomy. Relationship not only implies 

 common ancestry but likeness of structure, which is 

 more or less marked according to the degree of relation- 

 ship. But here we leave the region of fact for that of 

 inference. Palaeontology, in proportion to the com- 

 pleteness of the records in the rocks, speaks with a 

 certain voice as to, relationships. Similarity of structure 

 has to be sifted and checked in various ways before it 

 can be relied upon as evidence. We will take some 

 cases, beginning with extremes, to illustrate the matter. 

 The paca is a large brown rodent, with white spots. 

 The dasyure is an equally sized Australian marsupial, 

 also with a brown coat, diversified with white spots. 

 Why don't we, or do we, put together these two 

 animals ? We do not admit a close relationship for 

 the following reasons : In the first place white spots 

 on a brown or dark ground is a plan of coloration 

 found in many diverse kinds of animals between which 

 no intimate relationship is at all possible, which are 

 of course a priori conditions between two mammals. 

 Molluscs, insects, and other creatures have a host of 

 representatives which possess this character. It would 

 be tantamount to bracketing together green animals 

 and white animals, black creatures and transparent 

 creatures, a procedure which would involve too many 

 obvious absurdities to be entertained for a moment. 

 A second reason forbids us from laying any stress on the 

 white spots. In South America, where the paca lives, 

 there lives also the agouti. In the Australian continent 

 and its outlying islands, where we find the dasyure, 

 the Tasmanian devil is also to be met with. Now 

 neither the agouti nor the Tasmanian devil are brown 

 spotted with white ; and yet, in every other detail of 

 external and internal structure, the Tasmanian devil 



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