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In order to illustrate the difficulty referred to by Mr. Darwin, I will call 

 your attention to some instances in nature. 



Experiment has shown that the Zebra, the Hemionus, the Ass, and the 

 Horse, can mutually produce young, but that their produce, notwithstanding 

 certain exceptional instances of further fertility which have been sufficiently 

 well authenticated, cannot perpetuate themselves, and yet no naturalist holding 

 a position of eminence at the present day, would venture to deny that these 

 four races of animals have all descended from common ancestors. 



Here, then, we have an example of very limited divergence in outward 

 form, accompanied by great limitation in fecundity, and naturalists may, 

 notwithstanding their admitted descent from common ancestors fairly claim to 

 treat each of these animals as having reached the position of a separate 

 species. 



With the dog, on the other hand, although the external differences 

 between many forms, as, for example, the Bull dog, the Turnspit, and the 

 Greyhound, are far greater and more striking than those which we observe 

 between the Horse, the Ass, and the Hemionus, yet the former are always 

 perfectly fertile in interbreeding, and the cross-breeds perpetuate themselves. 

 Here then we have an example of considerable divergence in outward form, 

 in the " superficial characteristics " of Flourens, without any apparent inter- 

 ference with fecundity, and, yet, in this case, although naturalists have also 

 arrived at the conclusion that all existing varieties of the dog are descended 

 from common ancestors, they also treat each of these animals as a separate 

 species. 



If, therefore, continuous fecundity were the essential characteristic in the 

 determination of " species," then the Horse, the Ass, and the Hemionus ought 

 to be treated as separate species, whilst the Bull dog, and the Greyhound, and 

 all the other innumerable and peculiar forms of Dog found in every corner of 

 the globe, ought only to be ranked as varieties of one species. 



Such an adherence to any arbitrary rule is, however, unnecessary for 

 purposes of classification, though it bears strongly upon other points in the 

 theory propounded by Mr. Darwin to which I propose hereafter to call your 

 attention. But it is not only to animal life that the foregoing observations 

 extend. Although more difficult to understand in their application, the same 

 rules must be adopted in dealing with the classification of vegetable organisms. 

 Take, for example, plants belonging to the natural order Compositse which 

 includes the Daisy, the Groundsel, and other allied forms. Here on the table 

 you have four specimens, very similar in outward form, and at least as closely 

 allied in essential points of structure as the Horse and his congeners. 



Now, although we have not yet attempted to ascertain experimentally 

 whether these four forms would produce cross-breeds, I think few naturalists 

 would for one moment suppose that they would. If this be so, then we have 

 here an example of still greater divergence in fecundity, whilst we have no 

 difficulty in believing that these several forms, as well as all other plants 

 belonging to the same order, had a common origin. Indeed it would be easy 

 to bring together numberless examples from the book of nature, of incomplete 

 fecundity with slight divergence in outward form or general structure ; of 

 complete fecundity with great divergence in outward form ; and complete 

 sterility with great similarity in most of those characteristics, which are used 

 by naturalists for purposes of classification, while, at the same time, we should 

 have little hesitation in admitting the descent of all the species of each class, 

 from common ancestors. I will merely add further, that whilst all great 

 naturalists admit that it is quite chimerical to suppose that we can construct 

 any arrangement which shall be an absolutely correct expression of the plan of 

 nature, yet they also allow that we can, by carrying into effect with care and 



