of continual variation, revealing a relationship of descent between the different levels of the taxonomic hierarchy.
In the first portion of chapter two, Darwin sets up his argument by refuting the position of special creationists who believed that each species identified in the taxonomic hierarchy marked a unique creation that was readily identifiable by the existence of an indelible set of features. He argues that if this position is correct, then there should be a definite consensus about which organisms belong in a particular category. In order to test the veracity of this assumption, Darwin quantitatively compares the categorization statistics made by experts in the field, including H.C. Watson:
Compare the several floras of Great Britain, of France, or of the United States, drawn up by different botanists, and see what a surprising number of forms have been ranked by one botanist as a good species, and by another as mere varieties. Mr. H.C. Watson, to whom I lie under deep obligation for assistance of all kinds, has marked for me 182 British plants, which are generally considered as varieties, but which have all been ranked by botanists as species. . . . Under genera, including the most polymorphic forms, Mr. Babington gives 251 species, whereas Mr. Bentham gives only 112, - a difference of 139 doubtful forms! (41)
Using their own data, Darwin reveals that even experts in plant identification and categorization come to astonishingly little agreement about which organisms should be ranked as varieties and which as separate species. By casting doubt on the fixity of taxonomic categories, he creates an opportunity to present his own theories of evolution and natural selection, which he believes more adequately account for the data.
He opens the second portion of the chapter by clearly laying out his position:
Hence I look at individual differences, though of small interest to the systematist, as of high importance for us, as being the first step towards such slight varieties. . . . And I look at varieties which are in any degree more distinct and permanent, as steps leading to more strongly marked and more permanent varieties, and at these latter, as leading to sub-species and to species. . . . I attribute the passage of a variety, from a state in which it differs very slightly from its parent to one in which it differs more, to the action of natural selection in accumulating . . . differences of structure in certain definite directions. (44).
In these lines, Darwin presents a vision of diversity in nature as a dynamic process rather than as a static condition. He argues that the small differences observed in individual organisms can spread by descent throughout successive generations, making the offspring of those individuals slightly different from the general population from which they originated. These differences can widen through the continued accumulation of variation and eventually transform distinct varieties into distinct species. This dynamic process, Darwin argues, can be attributed to natural selection, which he defines as, “the preservation of favorable variations and the rejection of injurious variations” (68).
Once he has established his position on the source and character of diversity, Darwin presents arguments to attempt to link the size and range of a group of organisms at a particular level of the taxonomic hierarchy to the number of subordinate categories of organisms associated with that group. In order to do this, he depends both on arithmetical calculations using quantitative data and on the rhetorical/logical commonplace (topos) of the more and the less with which he establishes a connection between size/range of a population and the characteristic of diversity. In Book II of the Topics, Aristotle explains this strategy of argument: 7
Moreover, argue from greater and less degrees. There are four commonplace rules. One is: see whether a greater degree of the predicate follows a greater degree of the subject. . . . For if an increase of the accident follows an increase of the subject, as we have said, clearly the accident belongs; while if it does not follow, the accident does not belong. You should establish this by induction. (The Complete Works, 114b 35–115a 6)
Using statistics from available botanical compendia, Darwin calculates the number of botanical varieties belonging to species with the greatest estimated population sizes and ranges in hopes of discovering some general pattern in, or connection between, these species: “I thought that some interesting results might be obtained in regard to the nature and relations of the species which vary most, by tabulating all the varieties in several well-worked floras” (Origin 45).
To make his calculations, Darwin first divides the species in the compendia into large and small species according to the author’s size designations. He then divides the number of species in the large and small groups by the number of varieties that are connected with them to produce an average of the number of varieties for each species, large and small. The results of these tabulations and comparisons reveal that there is a correlation between the size and range of a species’ population and the number of varieties recorded for that species (Table 2).
Table 2. Arithmetical Comparisons of the Ratios between Species and Varieties in Large and Small Genera. Reprinted from Charles Darwin, Charles Darwin’s Natural Selection, Ed. R.C. Stauffer, p. 150. © 1975. Used by permission of the publisher, Cambridge University Press.
Based on this arithmetical comparison on the size of Genera and number of species, Darwin argues:
In any limited country, the species which are most common, that is abound most in individuals, and the species which are most widely diffused within their own country . . . often give rise to varieties sufficiently well-marked to have been recorded in botanical works. Hence it is the most flourishing, or, as they may be called, the dominant species . . . which oftenest produce well-marked varieties. (45–46)
What Darwin discovers, or confirms, as the result of his calculations, is that the more populous species tend to have a greater number of identified varieties associated with them. This correlation is accounted for by his dynamic theory of natural diversity because a correlation between the size of a population and the development of sub-populations would be expected as larger populations would have more offspring, and, therefore, a greater number of variations to be selected.
The correlation between the calculated size of a species and the number of varieties associated with it supports Darwin’s argument that diversity in nature is the result of the production of variations. In order to strengthen the conviction of the audience that this relationship is legitimate and to make the case that the relationship exists at all levels of the taxonomic hierarchy, Darwin predicts that the same relationship will be found between genera and species. To validate this prediction he conducts further calculations and comparisons to assess whether or not the principle holds true at the taxonomic level of genera. He walks his readers through his process of calculating and explains his results:
If the plants inhabiting a country and described in any Flora be divided into two equal masses, all those in the larger genera being placed on one side, and all those in the smaller genera on the other side, a somewhat larger number of the very common and much diffused or dominant species will be found on the side of the larger genera. (46)
Here, Darwin affirms his quantitative prediction that the same correlation between population size and variation which exists between species and varieties also exists between genera and species.
Strengthened by the predictive power of his model and the accumulating evidence, Darwin makes a point to emphasize the success of his theory in accounting for the patterns revealed by his calculations:
From looking at species as only strongly-marked and well-defined varieties, I was led to anticipate that the species of the larger genera in each country would oftener present varieties, than the species of the smaller genera; for wherever many closely related species . . . have been formed, many varieties or
