LAW OF SIMILARITY. 27 



diseases, as both defects and diseases appear to be more 

 easily transmissible than desirable qualities. There is 

 often no obvious peculiarity of structure, or appearance, 

 indicating the possession of diseases or defects which 

 are transmissible, and so, special care and continued 

 acquaintance are necessary in order to be assured of 

 their absence in breeding animals ; but such a tendency 

 although invisible or inappreciable to cursory observa- 

 tion, must still, judging from its effects, have as real 

 and certain an existence, as any peculiarity of form or 

 color. 



Every one who believes that a disease may be hered- 

 itary at all, must admit that certain individuals possess 

 certain tendencies which render them especially liable 

 to certain diseases, as consumption or scrofula ; yet it 

 is not easy^to say precisely in what this predisposition 

 consists. It seems probable, however, that it may be 

 due either to some want of harmony between different 

 organs, some faulty formation or combination of parts, 

 or to some peculiar physical or chemical condition of 

 the blood or tissues ; and that this altered state, con- 

 stituting the inherent congenital tendency to the dis- 

 ease, is duly transmitted from parent to offspring like 

 any other quality more readily apparent to observa- 

 tion. 



Hereditary diseases exhibit certain eminently charac- 



