Andingeneraleachoftheseanimalsnaturallytendstowardssterility。Theasshasallthedisadvantagesalreadymentioned，andifitshouldnotbegintogenerateafterthefirstsheddingofteeth，itnolongergeneratesatall；sonearistheconstitutionoftheasstobeingsterile。Thehorseismuchthesame；ittendsnaturallytowardssterility，andtomakeitentirelysoitisonlynecessarythatitsgenerativesecretionshouldbecomecolder；nowthisiswhathappenstoitwhenmixedwiththecorrespondingsecretionoftheass。Theassinlikemannercomesveryneargeneratingasterileanimalwhenmatedwithitsownspecies。Thuswhenthedifficultyofacrosscontrarytonatureisadded，（whentooevenintheothercasewhenunitedwiththeirownspeciestheywithdifficultyproduceasingleyoungone），theresultofthecross，beingstillmoresterileandcontrarytonature，willneednothingfurthertomakeitsterile，butwillbesoofnecessity。
Wefindalsothatthebodiesoffemalemulesgrowlargebecausethematterwhichissecretedinotheranimalstoformthecatameniaisdivertedtogrowth。Butsincetheperiodofgestationinsuchanimalsisayear，themulemustnotonlyconceive，ifsheistobefertile，butmustalsonourishtheembryotillbirth，andthisisimpossibleiftherearenocatamenia。Buttherearenoneinthemule；theuselesspartofthenutrimentisdischargedwiththeexcretionfromthebladder—thisiswhymalemulesdonotsmelltothepudendaofthefemales，asdotheothersolid—hoofedungulates，butonlytotheevacuationitself—andtherestofthenutrimentisuseduptoincreasethesizeofthebody。Henceitissometimespossibleforthefemaletoconceive，ashasbeenknowntohappenbeforenow，butitisimpossibleforhertocompletetheprocessofnourishingtheembryoandbringingittobirth。
Themale，again，maysometimesgenerate，bothbecausethemalesexisnaturallyhotterthanthefemaleandbecauseitdoesnotcontributeanymaterialsubstancetothemixture。Theresultinsuchcasesisa’ginnus’，thatistosay，adwarfmule；for’ginni’areproducedalsofromthecrossingofhorseandasswhentheembryoisdiseasedintheuterus。Theginnusisinfactliketheso—called’metachoera’inswine，fora’metachoerum’alsoisapiginjuredintheuterus；thismayhappentoanypig。Theoriginofhumandwarfsissimilar，forthesealsohavetheirpartsandtheirwholedevelopmentinjuredduringgestation，andresembleginniandmetachoera。
BookIII
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