in Zoology and Botany. 



189 



being extinct or unknown, I could point out numerous ex- 

 amples in which natural affinities are violated, insignificant 

 groups promoted, or important ones reduced to the ranks, in 

 the vain endeavour to drill the irregular troops of Nature into 

 the square, the column, and the phalanx*. And although in 

 some cases w T e do find examples of the recurrence of a certain 

 number in the subdivisions of natural groups, yet when we 

 remember the ease with which groups may be extended or 

 curtailed to support a theory, the numerous exceptions which 

 occur to these numbers, and the variety of numerical theories 

 which have been maintained with equal firmness by different 

 authors, we cannot, I think, regard these occasional coinci- 

 dences of number as otherwise than accidental. 



If, then, the diversities of organic structure, being adapted 

 to the varying conditions of the earth's surface, are, like them, 

 full of irregularity and variety, it is plain that we can no more 

 speculate theoretically as to what groups are likely to remain 

 undiscovered, than we can predict the discovery of rivers, 

 lakes or islands in any unexplored portion of the earth* s sur- 

 face. Both inquiries must be pursued in the same w 7 ay, viz. 

 by a careful induction of facts ; and it will be found that 

 there is much analogy between the process here recommended 

 and that of a geographical survey. The plan proposed is to 

 take any species, A, and ask the question, What are its near- 

 est affinities ? If, after an examination of its points of resem- 

 blance to all other known species, it should appear that there 

 are two other species, B and C, which closely approach it in 

 structure, and that A is intermediate between them, the ques- 

 tion is answered, and the formula B A C would express a por- 

 tion of the natural system, the survey of which is so far com- 

 pleted. Then take C, and ask the same question. One of 

 its affinities, that of C to A, is already determined ; and we 

 will suppose that D is found to form its nearest affinity on 

 the other side. Then B A C D will represent four species, 

 the relative affinities of which are determined. By a repeti- 

 tion of this process, supposing our knowledge of the structure 

 of each species to be complete, and our rules for determining 

 the degrees of affinity correct, the whole organized creation 

 might be ultimately arranged in the order of its affinities, and 

 our survey of the natural system would then be finally ef- 

 fected. Now, if each species never had more than two affi- 

 nities, and those in opposite directions, as in the above exam- 

 ple, the natural system would form a straight line, as some 

 authors have assumed it to be. But we shall often find, in 



* See Appendix. 



