2S cues i aa 
i 
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 we 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 way, 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 BAC 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. 
