Linnaean taxonomy

“ The system of binomial nomenclature has led to the development of the science of taxonomy, the systematic classification and naming of plants and animals according to their relationships.
THE UNITS OF CLASSIFICATION
The world of organic life has been divided into the plant and animal kingdoms. These kingdoms have been further divided into larger divisions called phyla (from the Greek word phylon, a race) . Each phylum is composed of organisms with certain characteristics in common. For example, all animals with a spinal cord (or notochord) are assigned to the phylum Chordata. ”

“ In many cases a Linnaean species has been split up till it became, first, a genus, then a family, and, in some cases, an order. What one naturalist considers a species is considered by another a genus containing several species, and, vice versa, the species of one authority is described as merely a variety by another. The older naturalists might have said with truth: we do not know what the species are, but we are quite certain that whatever they are they have never undergone any change in their distinguishing characters. ”

“ Yet we feel ourselves in some degree compensated for this serious error by his dividing trees and herbs into those with a two-leaved and those with a one-leaved or leafless embryo, in modern language into Dicotyledons and Monocotyledons. Ray's system is undoubtedly the one which in the time preceding Linnaeus does most justice to natural affinities. The following synopsis of his Classes will serve to show the progress made since Cesalpino. The names in brackets are the Linnaean names for some of the genera in particular classes. ”

“ Linnaeus' invention of binomial nomenclature for designating species served systematic biology admirably, but at the same time, by attaching preponderating importance to a particular grade in classification, crystallized the doctrine of fixity. The lower grades in classification such as sub-species and varieties on the one hand, and the higher grades on the other, such as genera and families, were admitted to be human conceptions imposed on the living world, but species were concrete, objective existences to be discovered and named. ”

“ It is equally impossible at present to distinguish different types of endemics, such as relictae, etc., as any such classification must await the elaboration of a method for determining the phylogeny of a natural group of species by an investigation of their comparative differentiation in connection with their migration in all directions from the vegetation center into new habitats. In short, it will not be possible to make a thorough study of endemism and to postulate its laws until modern methods of research have been extended to a much larger portion of the vegetation of the globe. ”

“ Critical field observations of plants in the wild state, supplemented by the cultivation side by side of species supposed to be distinct and by the lessons learned from experimental plant breeding, have developed the theory that many species, perhaps all, are composed of a greater or lesser number of races, differing from each other too little to cause them to be regarded as species, notwithstanding the fact that they may breed true from seed to such slight or trivial differentiations. ”

“ We know that it has been experimentally shown that a plot of land will yield a greater weight if sown with several species and genera of grasses, than if sown with only two or three species. Now, every organic being, by propagating so rapidly, may be said to be striving its utmost to increase in numbers. So it will be with the offspring of any species after it has become diversified into varieties, or subspecies, or true species. ”

“ Higher taxonomy data have been previously used to quantify species richness within specific areas by relating the number of species to the number of genera or families at well-sampled locations, and then using the resulting regression model to estimate the number of species at other locations for which the number of families or genera are better known than species richness (reviewed by Gaston & Williams [17] ) . This method, however, relies on extrapolation of patterns from relatively small areas to estimate the number of species in other locations (i.e., alpha diversity) . ”

“ What was more embarrassing, the number of distinct species went on increasing rapidly, indeed enormously, until, as an eminent zoological authority of our own time has declared, "for every one of the species enumerated by Linnaeus, more than fifty kinds are known to the naturalist of to-day, and the number of species still unknown doubtless far exceeds the list of those recorded." ”

“ One of the strongest arguments which have been adduced to prove the original and permanent distinctness of species is, that varieties produced in a state of domesticity are more or less unstable, and often have a tendency, if left to themselves, to return to the normal form of the parent species; and this instability is considered to be a distinctive peculiarity of all varieties, even of those occurring among wild animals in a state of nature, and to constitute a provision for preserving unchanged the originally created distinct species. ”

“ The constituting of classes, orders, and genera he calls theoretical classification; the constituting of species and varieties is practical classification. The work of classification carried out by Cesalpino, Morison, Tournefort, and others leads to the establishing of a system; the mere practice of describing species may be carried on by those who know nothing of systematic botany. These expressions of Linnaeus are interesting, because like other remarks of his they show that he placed the establishment and arrangement of the larger groups above the mere distinguishing of individual forms ”

“ For if the theory be sound, we should expect to find large numbers of species belonging to numerous and varied orders of plants presenting constant varieties on common areas; we should expect this to be a highly general, though not a universal, rule; and we should expect it to apply only to species which are indigenous. Moreover, we should expect these varieties, although but slightly differentiated morphologically, to present a great differentiation physiologically—and this in the special direction of selective fertility, combined, of course, with heredity. ”

“ Each new variety or species, when formed, will generally take the place of, and thus exterminate its less well-fitted parent. This I believe to be the origin of the classification and affinities of organic beings at all times; for organic beings always seem to branch and sub-branch like the limbs of a tree from a common trunk, the flourishing and diverging twigs destroying the less vigorous—the dead and lost branches rudely representing extinct genera and families. ”

“ It need not be supposed that all varieties or incipient species attain the rank of species. They may become extinct, or they may endure as varieties for very long periods, as has been shown to be the case by Mr. Wollaston with the varieties of certain fossil land-shells in Madeira, and with plants by Gaston de Saporta. If a variety were to flourish so as to exceed in numbers the parent species, it would then rank as the species, and the species as the variety ”

“ Hence during a long-continued course of modification, the slight differences characteristic of varieties of the same species, tend to be augmented into the greater differences characteristic of the species of the same genus. New and improved varieties will inevitably supplant and exterminate the older, less improved and intermediate varieties; and thus species are rendered to a large extent defined and distinct objects. Dominant species belonging to the larger groups within each class tend to give birth to new and dominant forms ”

“ As a rule, however, formations most frequently show endemic habitat forms and species, more rarely endemic genera; regions and provinces commonly exhibit endemic species and genera, rarely endemic families; while zones and hemispheres contain endemic orders as well as families. This correspondence is readily seen to depend primarily upon the fact that increased differentiation in the taxonomic sense is a concomitant of the increased invasion of endemic species, measured in terms of distance and difference in habitat. ”

“ Now this uncertainty is of little moment, when we have them primarily distinguished and set apart from other plants by their Monoecious or Dioecious character; because the genera being few, and the Orders constructed widely as to number of Stamens, we find little difficulty in determining any genus, which would be by no means the case if we had them confounded with the mass of the system. Even the species of the same genus, as well as individuals of each species, differ among themselves. ”

“ Some other basis for the classification of plants must therefore be chosen. More than a century ago, Linnaeus divided the vegetal kingdom into Cryptogamia and Phanerogamia, which may, for the present, be translated Flowerless and Flowering plants. Modern science has offered nothing better than the classification of Linnaeus, it being a natural one. The Flowerless plants, or the Cryptogamia, include: 1st, the Algae, or the greenish matter covering bricks, stones, etc., the green thread-plants of ponds and ditches, and the red and black sea-weed; 2d, the Fungi, or toadstools, mushrooms, etc. ”

“ Even on a wide area, which has during a long period remained continuous, and of which the climatic and other conditions of life change insensibly in proceeding from a district occupied by one species into another district occupied by a closely allied species, we have no just right to expect often to find intermediate varieties in the intermediate zones. For we have reason to believe that only a few species of a genus ever undergo change; the other species becoming utterly extinct and leaving no modified progeny. ”

“ To what extent these hints for the discrimination of species will be found to avail in practice, it is difficult to say, but it seems desirable that they should speedily be brought to the test of experience, since there is no genus, of the same extent, in which it is so difficult to determine the species. ”

“ For the full and legitimate fertilisation of these plants pollen from the one form must be applied to the stigma of another. If the sexual elements belonging to the same form are united the union is an illegitimate one and more or less sterile. With dimorphic species two illegitimate unions, and with trimorphic species twelve are possible. ”

“ In order to meet this difficulty we must recognize two sorts of species. The systematic species are the practical units of the systematists and florists, and all friends of wild nature should do their utmost to preserve them as Linnaeus has proposed them. These units however, are not really existing entities; they have as little claim to be regarded as such as genera and families. ”

“ Altogether there is the closest identity in character and behaviour between illegitimate plants and hybrids. It is hardly an exaggeration to maintain that the former are hybrids, but produced within the limits of the same species by the improper union of certain forms, whilst ordinary hybrids are produced from an improper union between so-called distinct species. We have already seen that there is the closest similarity in all respects between first illegitimate unions, and first crosses between distinct species. ”

“ Even with the individuals of the same sub-variety, when long kept by different fanciers, different strains can sometimes be recognised. There can be no doubt that, if well-characterized forms of the several Races had been found wild, all would have been ranked as distinct species, and several of them would certainly have been placed by ornithologists in distinct genera. A good classification of the various domestic breeds is extremely difficult, owing to the manner in which many of the forms graduate into each other ”

“ Because taxonomy is the foundation for much of the dialog and research in evolutionary biology, there must be an unambiguous taxonomic system in which one term is linked to one concept. In contrast to this ideal, homonymy and redefinition are prevalent in the taxonomy of eukaryotes, often as the result of premature introduction or redefinition of taxa (see above; Figure 3) . Emerging hypotheses benefit the community by sparking new research to test the hypothesis, but they also introduce ambiguity. ”

“ Again, we have seen that a tendency exists in nature to eliminate hybrid races, by whatever means that elimination is effected, while no similar tendency bars the way to an indefinite blending of varieties. This has also been enforced by statements as to the prepotency of certain pollen of identical species, but of distinct races. ”

“ This is evident when it is considered that fluctuations can only appear with their pure and normal type in pure strains, and that each admixture of other units is liable to be shown by the form of the curves. More over, selection chooses single individuals, and a single plant, if it is not a hybrid, can scarcely pertain to two different species. The first choice therefore is apt to make the strain pure.
In contrasting selection between species with that within the species, of course elementary species are meant, including varieties. ”