116 THE ORIGIN OF SPECIES 



is distinguished from all others in the world by the follow- 

 ing constantly associated characters. They have 1, A 

 vertebral column ; 2, Mammae ; 3, A placenta! embryo ; 4, 

 Four legs ; 5, A single well-developed toe in each foot 

 provided with a hoof ; 6, A bushy tail ; and 7, Callosities 

 on the inner sides of both the fore and the hind legs. The 

 asses, again, form a distinct species, because, with the same 

 characters, as far as the fifth in the above list, all asses 

 have tufted tails, and have callosities only on the inner 

 side of the fore-legs. If animals were discovered having 

 the general characters of the horse, but sometimes with 

 callosities only on the fore-legs, and more or less tufted 

 tails ; or animals having the general characters of the ass, 

 but with more or less bushy tails, and sometimes with 

 callosities on both pairs of legs, besides being intermediate 

 in other respects the two species would have to be merged 

 into one. They could no longer be regarded as morpho- 

 logically distinct species, for they would not be distinctly 

 definable one from the other. 



However bare and simple this definition of species may 

 appear to be, we confidently appeal to all practical 

 naturalists, whether zoologists, botanists, or palaeontologists, 

 to say if, in the vast majority of cases, they know, or 

 mean to affirm, anything more of the group of animals or 

 plants they so denominate than what has just been stated. 

 Even the most decided advocates of the received doctrines 

 respecting species admit this. 



" I apprehend," says Professor Owen,* " that few naturalists 

 nowadays, in describing and proposing a name for what they call 

 ' a new species,' use that term to signify what was meant by it 

 twenty or thirty years ago ; that is, an originally distinct creation, 

 maintaining its primitive distinction by obstructive generative 

 peculiarities. The proposer of the new species now intends to state 

 no more than he actually knows ; as, for example, that the differ- 

 ences on which he founds the specific character are constant in 

 individuals of both sexes, so far as observation has reached ; and 

 that they are not due to domestication or to artificially superin- 

 duced external circumstances, or to any outward influence within 

 his cognizance ; that the species is wild, or is such as it appears by 

 Nature." 



If we consider, in fact, that by far the largest proportion 

 of recorded existing species are known only by the study 



* On the Osteology of the Chimpanzees and Orangs : Transactions 

 of the Zoological Society, 1858. 



