406 Mr. W. S. MacLeay's Examination of 



clearly the essential characteristic laid down in the law, since a Negro 

 " produces by certain laws of generation others like himself," and yet 

 is not very generally accounted to be a distinct species. But I ought to 

 recollect, that in spite of Mr. Wilberforce, you have yoiir doubts on this 

 particular point ; that in fact it still remains with you " the most difficult 

 " problem of all." 



You lay down as a '* first principle of arrangement," that " in Botany 

 " the characters of a genus should be taken from the parts of fructifica- 

 *' tion, and in Zoology from such parts as are indicative of structure and 

 " habits." Having myself, as you know, dabbled a little in Zoology, 

 and being pleased with the sight of a really new definition, I am anxious 

 to learn what other zoological parts remain, in order that I may avoid 

 them. 



To clear up the fog in which our poor brains are enveloped when we 

 attempt to distinguish a species from a genus, you next inform us that 

 " there is the same difference between a genus and a species as instru- 

 " ments of reasoning, as between a definition and a proposition in Geo- 

 " metry. Now the difference between the latter is, that the proposition 

 requires demonstration, and the definition not. I must therefore sup- 

 pose that this mode of illustration is " wi lucus a non lucendo,'' for you 

 have just before declared that species must " be brought to the test of 

 " experiment," in other words, must be demonstrated. 



It appears you do not regard genera as merely conventional, but as 

 actually founded in nature as well as species. I likewise consider genera 

 when properly defined, to be founded in nature, as I have elsewhere 

 said,* but I have not found even these natural genera, upon the whole, 

 to be so distinctly insulated from each other as species. I will now, 

 however, go further than you, by stating that the groupes you object to, 

 such as class, order, tribe, cohort, and family, are, when properly 

 defined, just as natural as genera ; and also that the higher we ascend in 

 the scale, and the more comprehensive our groupes are, we may, in gene- 

 ral, be assured, that in the same proportion they are perhaps even more 

 natural. Thus, who will assert that Animals form a less natural groupe 

 than Vertehrata, Vertebrata than Mammalia, Mammalia than Cetacea, or 



♦ See Horae Ent. p. 490. 



