THE MULES. 335 



where all the conditions of the problem are not 

 proved to be similar, they exceed the proper limits 

 of inference, as we have already shown in the x^^atu- 

 ral History of Dogs, and endeavoured again to point 

 out in the foregoing pages. The laws affecting 

 organic matter are modified by the Power that 

 ordained them, and subjected to a multitude of 

 exceptions, warning us at every moment to be cau- 

 tious in the assignment of their bounds. Formerly, 

 because science would not recognize the evidence of 

 these modifications, it was endeavoured to escape 

 from acknowledging the value of truth, by asserting 

 that bats were birds and cetacea fishes, because they 

 were not quadrupeds ; and when the objection was 

 destroyed by adopting as a general term the word 

 mammalia, many, habituated to received doctrines, 

 maintained them to be at best on the utmost verge 

 of possible adaptations of that class of beings ; but 

 with a more intimate knowledge of American ani- 

 mals, and still more after the discovery of the 

 Marsupialia of New Holland, new phenomena in 

 gestation and reproduction came to light. In the 

 case of Opossums, they had often been denied or 

 overlooked, and were held impossibilities, until sys- 

 tematic research overthrew all doubt and transferred 

 incredulity to the as yet unsettled questions relating 

 to the Monotremes, whose wonderful history is con- 

 spicuous in the Ornithorynchus or water-mole. 



Now, all these questions were and are accessible 

 to direct proof by anatomical investigation ; and if 

 they were so long contested more than examined, 



