species – those that range most extensively over the world, are the most diffused in their native regions, and are the most numerous – are the ones that most often produce varieties, or as I consider them, incipient species. This too might have been anticipated, because for a variety to become permanent, it must struggle with other inhabitants of the region. Already dominant species are the most likely to leave offspring, which, in addition to being slightly modified, will inherit those advantages that enabled their parents to become dominant.
If the plants of a particular region are separated into two groups, with those belonging to large genera in one and those belonging to small genera in the other, then a greater number of dominant species will be found on the side of larger genera. Again, this might have been anticipated: if many species of a genus inhabit a region, it shows that something about the organic or inorganic conditions of that region are favorable to that genus; therefore a large genus encompassing many species contains a proportionally greater number of dominant species. But so many factors tend to obscure this result that I am surprised my tables show even a small majority of dominant species among large genera. To give examples of how complex this is, consider that freshwater and salt-loving plants are generally diffused with extensive ranges, but this is probably because of the nature of their habitats rather than the size of the genera to which they belong; likewise, the wide distribution of simple plants is also unrelated to genera size. (The reason that simple plants range extensively will be discussed in the chapter on geographic distribution.)
Because I think of species as being just well-defined varieties, I anticipated that species from a large genus would tend to have more varieties than species from a small genus: if many closely related species (i.e., members of a genus) have already formed, many incipient species should still be forming. Where many large trees grow, we expect to find saplings. Where many species within a genus have arisen through variation, circumstances have favored variation, and we expect them to still favor variation. However, if we consider each species a special act of creation, then there is no apparent reason to expect more varieties in a large genus than in a small one.
To test this I arranged the plants of twelve countries and the coleopterous insects of two districts into two groups with, again, species from larger genera on one side and those from smaller genera on the other. The species in the group of large genera invariably have a greater proportion of varieties than species in the group of small genera. Moreover, species of large genera with any varieties invariably have a higher average number of varieties than species of small genera. The same results follow when the division is made after excluding genera with four or fewer species. These observations are understandable if species are just well-defined and permanent varieties, because wherever many species within a genus have been generated, the generation should still be happening, especially because the process of species formation is probably slow. This is in fact the case if varieties are considered incipient species. This is not to say that all large genera vary greatly, their ranks now swelling with species; moreover, some small genera do vary and are increasing. It would be fatal to my theory if this were not so, because geology plainly reveals that small genera have often increased greatly and large genera have peaked, declined, and disappeared.3 All I want to show is that if many species have been formed within a genus, on average, many are still forming – and this is supported by my data.
Again, there is no infallible criterion by which a species can be distinguished from a well-defined variety, and if intermediate links between doubtful forms cannot be found, then naturalists are compelled to come to a determination based on the amount of difference between them and judge by analogy whether it suffices to rank one, the other, or both as species. Fries has remarked with respect to plants and Westwood with respect to insects that in large genera the differences between species are exceedingly small. I have tried to test this numerically by averages, and as far as my imperfect results go, they always confirm it. I also consulted several knowledgeable and experienced observers who, after deliberation, concurred. Therefore, species in large genera resemble varieties more than do the species of small genera. Put another way, large genera with many incipient species contain already-generated species that resemble varieties because they do not differ from one another extensively.
Moreover, species within a large genus are related to one another in the same way as varieties within a species. Of course, species within a genus are not all equally distinct from one another; they can generally be divided into sub-genera, sections, or lesser groups. Fries has correctly remarked that little groups of species cluster like satellites around certain other species, and what are varieties but unequally related groups of forms clustered around their parent species? There is one important difference between varieties and species: the amount of difference between varieties when compared to one another or their parent species is less than the amount of difference between species within a genus. This will be explained in the discussion of what I call “divergence of character,” along with the tendency of varietal differences to burgeon into the greater differences between species.
I think one other point is worth noting. Varieties generally have restricted ranges (although this is a truism, for if a variety were found to have a greater range than its parent species, their denominations would be reversed). But species that are related to many other species, and therefore resemble varieties, also tend to have restricted ranges. For example, Mr. H. C. Watson has identified for me sixty-three species-ranked plants in the fourth edition of the well-sifted London Catalogue of Plants that he considers so closely related to other species as to doubt their assigned rank. These sixty-three supposed species range on average over 6.9 of the provinces into which Mr. Watson has divided Great Britain. The same catalog lists fifty-three acknowledged varieties that range over 7.7 provinces, whereas the species to which they belong range over 14.3 provinces. This means that the acknowledged varieties have almost the same average range as the forms Mr. Watson identifies as doubtful species but that British botanists almost universally mark as true species.
Finally, then, varieties and species have the same general properties and cannot be distinguished except by (1) the discovery of intermediate links, although such links do not affect the characteristics of the forms they connect, and (2) a certain amount of difference that cannot be exactly defined. Within any given region, genera with a greater than average number of species also have species with a greater than average number of varieties. In large genera, species tend to be closely but unequally related and clustered around certain species. Species very closely related to other species apparently have restricted ranges. In all of these respects, species of large genera are like varieties. These patterns can be clearly understood if species were once varieties, but they are completely inexplicable if each species has been independently created.
Also, on average, flourishing and dominant species of large genera vary the most, and varieties, as discussed later, tend to become converted into new and distinct species. Large genera thus tend to become larger and dominant forms become more dominant as they leave many modified and dominant descendants. Yet by steps explained later, large genera also tend to break up into smaller genera. And so it is that the life forms of the universe become divided into groups subordinate to groups.
1. The sound opinion and wide experience of naturalists seems the only guide to follow in determining whether a form should be ranked as a species or a variety. However, in many cases the best course is to follow the majority, because there are very few well-defined and known varieties that have not also been listed as species by at least some qualified judges.
2. Being “diffused” is a different consideration from “extensive range” and “commonness.”
3. [Note that in Darwin’s time, geology encompassed what we would today recognize as paleontology. – D.D.]
3
