My dear Waterhouse

Now for a letter in answer to your two ones on classification—on which I have been often thinking. It has long appeared to me, that the root of the difficulty in settling such questions as yours,—whether number of species &c &c should enter as an element in settling the value or existence of a group—lies in our ignorance of what we are searching after in our natural classifications.—¹ Linnæus confesses profound ignorance.—² Most authors say it is an endeavour to discover the laws according to which the Creator has willed to produce organized beings— But what empty high-sounding sentences these are— it does not mean order in time of creation, nor propinquity to any one type, as man.— in fact it means just nothing—³ According to my opinion, (which I give every one leave to hoot at, like I should have, six years since, hooted at them, for holding like views) classification consists in grouping beings according to their actual relationship, ie their consanguinity, or descent from common stocks— In this view all relations of analogy &c &c &, consist of those resemblances between two forms, which they do not owe to having inherited it, from a common stock.—⁴ To me, of course, the difficulty of ascertaining true relationship ie a natural classification remains just the same, though I know what I am looking for.— This being the case viz ignorance of a distinct object I think, we ought to look at classification as a simple logical process, i.e. a means of conveying much information through single words— now the laws of classification tryed by this rule—it is clear that neither number of species—nor grade of organization ought to come in, as an element.— For instance, the word Marsupiata implies a great number of facts, common to many animals, and if by introducing another group of animals the definition of Marsupiata is forced to include, any one fact, not applicable to the former animals, then I maintain the second group ought not to have been added to the first.⁵ But if they have much in common, a third more general term ought to be invented to include them.— Viewing classification, as a logical process, the only cheque to the splitting of groups, appears to be convenience; & if you had objected to making the Monotremes & the Styloptera⁶ orders, on account of the inconvenience of increasing the number of orders I think, it might have been valid. I repeat, that until you can define your object in classifying, you have no right to introduce number of species, or (arbitary) grades of organization, but are bound to follow simple amount of differences of organization.— This, I think, you will find is the rule amongst Botanists, & they have many families very small in number.— This is clearly the rule, on the view of classification, being a genealogical process, exhibiting literal or actual relationship.

There is one caution, which should not be overlooked, namely the great doubt whether the groups, which are now small, may not have been at some former time abundant: and you will admit fossil & recent beings all come into one system.— In fish, it would appear, that some of the main divisions, which are now least abundant in species, appear formerly to have been most so. It w^d take a Chapter to argue, how probable it is that Geology has never revealed & never will reveal, more than one out of a million forms, which have existed. Finally, however, I confess, there is much weight in your argument, that as most orders contain many species, the usual harmony of nature would make you very cautious in admitting a few species to form a group of equal rank with one, containing many species— It is certainly a very curious fact, the existence of only a few groups, apparently of high value & containing only a very few species.— The only doubt, which has ever occurred to me, lies in the rules being so exceedingly arbitary by which the value of groups are judged by. I mean, that perhaps many orders really exist, containing only one or two genera, but that from this very circumstance, they are not viewed as orders but only as families— Perhaps if the Goatsucker & Woodpecker, were varied into very many genera & very many species of each—they would be looked on as orders equal to the Hawks &c &c— (Tell me what you think of this) This is a mere illustration of what I mean.—

—Finally then I protest against number or grades of organization being used as elements in classification, though I believe they have silently been used.—

Have you had patience to read thus far.— Yours ever, C. D.