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
THE STRUGGLE FOR EXISTENCE
BEFORE DISCUSSING THE STRUGGLE FOR EXISTENCE, I NEED to show how it is relevant to natural selection. As mentioned in the previous chapter, individual organisms in the wild vary from one another. (I am not aware that this has ever been disputed.) It is not important whether a multitude of doubtful forms are called “species,” “sub-species,” or “varieties.” For example, what rank the two or three hundred doubtful British plants are entitled to hold is immaterial if the existence of any well-marked varieties is accepted. The existence of individual variability and of well-marked varieties is a necessary foundation, but does not help explain how species arise. How have all the exquisite adaptations of one part of the organization to another and of living things to their environments – including other living things – been perfected? We see beautiful coadaptations in the woodpecker and mistletoe, in the humblest parasite that clings to the hairs of a quadruped or the feathers of a bird, in the structure of a beetle that dives through water, and in the plumed seed that is wafted by the gentlest breeze. In short, we see beautiful adaptations in every part of the organic world.
How are varieties – incipient species – converted into distinct species? How do groups of species making up distinct genera arise? All of these results follow from the struggle for life. Any variation, regardless of magnitude or ultimate cause, that is profitable to an individual through its complex relationships with other organisms and nature increases that individual’s chances of survival and will generally be inherited by its offspring. These offspring will consequently also have a better chance of surviving, because only a small number of individuals that are born can actually survive. I call this principle, by which each slight useful variation is preserved, “natural selection” to mark its relation to the human power of selection. I have shown that by selection, humans can produce great results and adapt organisms to their own uses by accumulating slight but useful variations provided by nature. But as I will show, natural selection is a power that is always in action and is as immeasurably superior to feeble human efforts as the works of nature are to those of art.
Sir Charles Lyell and the elder de Candolle have shown that all organisms face severe competition. With respect to plants, W. Herbert, Dean of Manchester, has treated this subject outstandingly, evidently the result of his great horticultural knowledge. Nothing is easier than recognizing the universal struggle for life, but nothing is more difficult than constantly keeping it in mind. Yet unless it becomes thoroughly in-grained, I am convinced that the whole economy of nature – distribution, rarity, abundance, extinction, and variation – will be seen only dimly or misunderstood. We behold the face of Nature bright with gladness and often see a superabundance of food, but we forget that the birds singing idly around us live on insects or seeds, thus constantly destroying life; we forget how frequently these songsters, their eggs, and their nestlings are destroyed by birds or other predators; we forget that although food may be plentiful now, it is not so in all seasons or in all years.
I use the term “struggle for existence” broadly and metaphorically to include the dependence of organisms on one another, and, more importantly, the success an individual has in leaving progeny. In a time of want, two dogs may truly be said to struggle with each other over which shall get food and live. But a plant on the edge of a desert struggles against drought; it is dependent on moisture. A plant that produces a thousand seeds a year, of which on average only about one will mature, struggles against members of its own species and other plants that already clothe the ground. The mistletoe is dependent on the apple and a few other trees but does not exactly struggle with them – yet if too many of these parasites were to cover one tree, it would languish and perish. However, several seedling mistletoes growing together on one branch do struggle with one another. The mistletoe struggles metaphorically against other fruit-bearing plants in tempting birds that