Читать книгу Darwin's On the Origin of Species - Daniel Duzdevich - Страница 13

BEFORE APPLYING THE PRINCIPLES FROM THE LAST CHAPTER to living things in the wild, we need to establish whether or not they too are subject to variation. A proper treatment of this topic would involve a long catalog of dry facts, but I will reserve this for my future work. And I won’t discuss the various definitions of “species,” because no one definition satisfies all naturalists, even though everyone vaguely knows what it means. Generally, the term includes an unknown element of a distinct act of creation. “Variety” is almost as difficult to define, but in this case community of descent is often implied even though it can rarely be proven. There are also monstrosities – by which I mean considerable structural deviations that are either harmful or useless to the species and not usually propagated – but these graduate into variations. Some authors use the term “variation” in a technical sense to indicate an un-inheritable modification resulting from environmental conditions, but the dwarfed shells of brackish Baltic waters, dwarfed plants of alpine summits, and the thickened fur of animals living far north might be inherited for at least a few generations, and in such cases I would call the form a variety.

Once again, consider that there are many slight individual differences, such as those observed in offspring from the same parent. (Sometimes differences are just assumed to have arisen this way because they are frequently observed in individuals of the same species living in a confined locality.) No one would argue that all the individuals of a species are cast in the very same mold. Individual differences are important because they supply material for natural selection to accumulate, just as humans can accumulate individual differences in any direction in domestic organisms. Individual differences generally occur in what naturalists consider “unimportant parts,” but I could show a long list of examples where physiologically or classificatorily important parts also vary. I am certain that the most experienced naturalist would be surprised at how many cases of variability, even in important structural parts, can be collected from good authorities. Keep in mind that systematists do not like finding variations of important characteristics, and there are not many people prepared to laboriously compare the internal organs of many specimens from one species. I never expected that the branching pattern of nerves close to an insect’s central ganglion would vary within a species – I expected such changes to occur in small increments – yet Mr. Lubbock recently showed the variability of these main nerves in Coccus to be comparable to the irregular branching of trees. He also found that the muscles in the larvae of certain insects are far from uniform. Some authors state that important organs never vary, and in a circular argument they basically classify “important traits” as those that are invariable (a few naturalists have honestly confessed this). In this scheme there cannot be any varying important parts by definition! But under any other scheme many examples can be given.

With respect to individual differences, one point strikes me as extremely perplexing: the “polymorphic” genera, in which species vary so inordinately that few naturalists can agree which forms should be ranked as species and which as varieties. Among plants, examples include Rubus, Rosa, and Hieracium; among animals, there are several genera of insects and brachiopods. Most polymorphic genera contain some species with fixed characteristics. Also, genera that are polymorphic in one region seem to be polymorphic in others, with a few exceptions. Judging from brachiopod shells, genera remain polymorphic through time. These observations are perplexing because they suggest that this type of variability is independent of the environment. I suspect that members of polymorphic genera vary in characteristics that are of no service or disservice and have therefore not been seized and rendered definite by natural selection, as I explain later.

Those forms that could justifiably be considered species but are similar enough to other forms or linked to them by intermediates so that naturalists do not rank them as distinct species are very important for this discussion. There is every reason to believe that many of these “doubtful” forms have permanently retained their characteristics in their native regions for as long as “true” species. On a practical level, when a naturalist can link two forms through intermediates, he will treat the most common one, or the one that happened to have been described first, as the species and the other as a variety. But there are some very challenging cases – I won’t list them – concerning whether or not one form should be classified as a variety of another even if they are closely linked by intermediates. The commonly assumed “hybrid nature” of intermediates does not always solve the problem. In many cases the intermediate forms have not been found, and they are assumed by analogy to either exist undiscovered or to have become extinct; here a wide door for the entry of doubt and conjecture is opened.

It cannot be disputed that these doubtful forms are common.¹ A surprising number of plants from Great Britain, France, and the United States have been ranked as species by some botanists and mere varieties by others. Mr. H. C. Watson, to whom I am grateful for all kinds of assistance, has listed for me 182 British plants that are generally considered varieties but have all been ranked by some botanists as species. He did not include many minor varieties that have nevertheless been ranked as species by some botanists; he entirely omitted several highly polymorphic genera. Under genera, Mr. Babington lists 251 species, whereas Mr. Bentham lists only 112, amounting to a difference of 139 doubtful forms! Doubtful forms of mobile animals that mate to reproduce are rare within the same region but common in separated areas. Many slightly differing North American and European birds and insects are ranked by one naturalist as undoubted species and by another as varieties. Many years ago, when comparing birds of the Galápagos Archipelago to one another and to those of the American mainland, I was struck by the vague and arbitrary distinction between species and varieties. Many of the insects on Madeira are ranked as varieties in Mr. Wollaston’s admirable book but would certainly be characterized as species by many entomologists. Even Ireland has a few animals generally considered varieties but ranked as species by some zoologists. Several experienced ornithologists consider the British red grouse a variety of a Norwegian species, while most rank it as a species peculiar to Great Britain. If there is a large distance between the homes of two doubtful forms, many naturalists rank them as species, but what distance will suffice? If that between Europe and America is enough, is the distance between the Continent and the Azores, or Madeira, or the Canaries, or Ireland also enough? Trying to discuss the demarcation between a variety and a species before establishing any definition of these terms is like hammering the air.

Several interesting arguments from geographic distribution, analogical variation, hybridism, and so forth have been developed to address this problem with respect to different cases. I will discuss only the example of the primrose and cowslip. These plants differ in appearance, flavor, odor, flowering period, and range. They grow in somewhat different habitats and ascend mountains to different heights. Finally, according to many experiments conducted over the course of several years by the careful observer Gärtner, they can be crossed only with great difficulty. There could hardly be better evidence that two forms are distinct species. But for all that, they are linked by many intermediates that are probably not hybrids. There is also overwhelming experimental evidence showing that they have descended from a common parent and are therefore varieties.

In most cases, close investigation brings naturalists to agree on how to rank a doubtful form. Yet the greatest number of doubtful forms is found in the best-known regions. I have noticed that if any plant or animal is useful or interesting to humans, varieties of it will be found recorded; these varieties, moreover, will be ranked by some authors as species. Consider the common oak, so closely studied. A German author makes more than a dozen species out of forms generally considered varieties. And in this country some of the highest botanical authorities claim sessile and pedunculated oaks as species while others claim them as varieties.

When a young naturalist begins studying an unfamiliar group of organisms, he will find it difficult to determine differences that delineate species and those that delineate varieties, because he doesn’t yet know the amount and kind of variation within the group. (This demonstrates, at least, that there is often some variation.) But if he confines his attention to one class in one region, he will soon make up his mind about how to classify doubtful forms. He will define many species because the differences impress him, and he lacks a general knowledge of analogical variation in other groups and other countries. As he extends his range of observation, he will discover more cases that are difficult to classify, for he will encounter more forms that are closely related. If his observations are extended further, he will ultimately make up his mind about species and varieties, but only at the expense of admitting extensive variation – an admission other naturalists will dispute. When he comes to study related forms from regions that are no longer continuous, and where intermediate links between doubtful forms will necessarily be absent, he will have to trust almost entirely to analogy and his difficulties will rise to a climax.

No clear line has been drawn between species and sub-species (forms that are nearly ranked as species but not quite there), sub-species and varieties, or lesser varieties and individual differences. They all blend into each other in a smooth series, and a series impresses the mind with the idea of an actual progression.

Although individual differences are unimportant to the systematists, they are very important to this argument as the first step toward the minor varieties barely thought worthy of notice in annals of natural history. I consider even slightly distinct and permanent varieties as steps leading to still more distinct and more permanent varieties, and these in turn as leading to sub-species and species. Progression from one stage to another may in some cases be due merely to the continuous and long-term action of different environmental conditions in two different regions, but I have little faith in this view. Instead, when a variety changes from differing little from its parent to one that differs more, I attribute the change to natural selection having accumulated structural differences in certain definite directions. Therefore, I believe a distinct variety can be called an incipient species. Whether this is justified should be judged by the general weight of observations and ideas presented throughout this book.

Not all incipient species actually become species. They may become extinct in the incipient state or simply remain varieties for very long periods (as Mr. Wollaston has shown for certain mollusk varieties based on fossil land shells in Madeira). If a variety were to flourish and exceed its parent species in number, it would be ranked as a species and the parent as a variety; it might supplant and exterminate the parent or coexist with it, both being ranked as independent species. (I will return to this subject.)

From these considerations I take the term “species” as being arbitrarily assigned for the sake of convenience to groups of individuals closely resembling one another. It does not essentially differ from the term “variety,” given to less distinct and more fluctuating forms. “Variety” is also a term that is applied arbitrarily and for convenience, again, in comparison with mere individual differences.

Guided by theoretical considerations, I thought that tabulating all the varieties of several well-studied plants would yield some interesting data about the nature of species that vary the most. At first this seemed simple, but Mr. H. C. Watson and Dr. Hooker soon convinced me of the many difficulties. (I am reserving the tables themselves and a discussion of these difficulties for my later work.) After carefully reading my manuscript and examining the tables, Dr. Hooker agreed that the following statements are fairly well established. Nevertheless, the subject is complex and treated tersely here, so allusions cannot be avoided to “the struggle for existence,” “divergence of character,” and other concepts that will be discussed later.

Alph. de Candolle and others have shown that plants with extensive ranges generally have varieties. This might have been expected because it means exposure to diverse physical conditions and competition with different sets of organisms (competition is by far the more important, as discussed later). My tables also show that in a limited region the most common species (those with the greatest number of individuals) and the species that are most diffused often have varieties well-defined enough to be recorded in botanical works.² Therefore the most “dominant” 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.]