The Search for New Systems of Classification

Overview

As naturalists began to expand their reach and study more and more of the Earth, they began to find such a bounty of plants and animals that the current and relatively informal practice of naming new discoveries quickly became outdated. Botanists and zoologists found themselves overwhelmed and unable to keep track of the different organisms. By the 1700s the problem had become severe. Swedish naturalist Carolus Linnaeus (1707-1778) recognized the need for a clear and logical method of classifying and naming living things. Thus, in his Systema naturae (1735) he introduced the practice of using two Latin names—one for genus, the other for species—to identify each different plant and animal known to man. This method, termed binomial nomenclature, is still used today. Baron Georges Cuvier (1769-1832) further refined the system of classification by providing a description for so-called "type" animals that are representatives of each species.

Background

The 1700s saw a dramatic rise in the number of discoveries of living things. Botanists in particular introduced new species almost daily. At that time, however, the naming of plants was a very inexact science without universal guidelines. The result was a confusing tangle of plant names that made it difficult, if not impossible, both to determine how a new plant compared to other varieties and to simply ascertain whether a "new" plant had really already been discovered and given a different name by another botanist. The same situation was faced by zoologists and animal species.

Linnaeus recognized the depth of the problem early in his career. With only four years of formal higher education behind him, he went to Lapland to conduct a five-month survey of its plants, animals, and minerals. During that five-month survey he wrote detailed descriptions of all of the organisms he encountered. Upon his return, as he summarized his findings and discoveries, he also drafted the first edition of Systema naturae.

The first edition of Systema naturae was a 15-page publication that presented a preliminary view of his thoughts on classification. Over the years, he published subsequent and much longer editions, including what would become the most influential—the 1,300-page 10th edition in 1758. In all, he published 12 editions. Under Linnaeus's system, each organism receives a generic (genus) and specific (species) name. The generic name helps botanists and zoologists to determine which organisms are closely related. Two species of cattails, for instance, share the genus Typha. The specific name is unique to that organism. The common cattail's specific name is latifolia, whereas the narrow-leaved cattail carries the specific name of augustifolia. Under the two-name system, or binomial nomenclature, the common cattail's scientific name, then, is Typha latifolia, while the narrow-leaved cattail is Typha augustifolia. (The genus name is often abbreviated to its first initial in subsequent references, such as T. latifolia and T. augustifolia.)

Linnaeus also developed higher levels of organization—with kingdom at the top, then class, order, and finally genus and species. He described three kingdoms: one for animals, one for plants and one for minerals. Each kingdom contained classes. His original classes under the animal kingdom, for example, included Amphibia (amphibians), Aves (birds), Insecta (insects), Mammalia (mammals), Pisces (fish), and Vermes (worms). While he arranged animals with an eye toward morphological similarities, he classified plants based on their flowers and other parts of their reproductive systems, such as the stamens and pistils.

Other eighteenth-century scientists also made important contributions to the classification system. French zoologist Cuvier introduced to Linnaeus's animal hierarchy the phylum level, which falls between kingdom and class. His four phyla included Vertebrata (animals with a backbone), Mollusca (including clams and squid), Articulata (including worms and insects), and Radiata (including sea anemones and jellyfish). Scientists Michel Adanson (1727-1806) and Georges Buffon (1707-1788), who was actually a critic of Linnaeus, furnished the classification system with the level of family between genus and order.

Cuvier also developed the concept of "types" to represent animal species. One particular species of grasshopper, then, would have one individual—the "type"—to represent and describe the entire species. That type would carry the species' different characteristics.

Impact

The classification system introduced by Linnaeus in the 1700s has shaped the way botanists and zoologists view the natural world. Linnaeus brought logic and organization to the naming of plants and animals, and also put in place a taxonomic system that encouraged scientists to look for similarities and differences among organisms.

In his Systema naturae Linnaeus described kingdom, class, order, genus, and species. All of these levels of hierarchy remain in use today, along with many of the titles he assigned. His original class names of Mammalia, Aves, Amphibia, and Insecta, along with his kingdoms of Plantae and Animalia, have survived the test of time. Scientists have since more than quadrupled the number of classes, and added many, many genera to Linnaeus's original list. His classification system, however, was so versatile that it allowed for this massive growth.

With his system—along with the additions in hierarchy levels, such as the family and phylum—scientists can now understand a great deal about an animal or plant just by knowing where it is placed taxonomically. For example, a new species can usually be immediately identified as belonging to only one of the multitude of orders. All frogs and toads, for example, fall under Order Anura. All members of the Order Anura share certain characteristics, so although a zoologist may not have done a complete study of that particular organism, he or she can initially assume that the specimen shares the order's characteristics, such as the presence of a shortened backbone or the use of external fertilization in reproduction. By noting other characteristics, which are often readily apparent, scientists can then quickly place the plant or animal in the correct family. For example, members of the Bufonidae (true toads) family of the Order Anura have thick, warty skin, and likely lay their eggs in long strings.

As this process of deduction continues, the identification follows down to the species level. Species are defined as groups of organisms that breed with one another and produce viable offspring. In all of the taxonomic levels, only the species is a natural unit. The others—from genus all the way to kingdom—are artificially created groupings formed to help scientists wend their way through the tangle of living things.

Zoologists and botanists officially accepted Linnaeus's classification system in the early twentieth century. Botanists regard Linnaeus's 1753 Species plantarum as the starting point for plant naming, and zoologists chose his 1758 tenth edition of Systema naturae as the starting point for animal names. For animals, the official recognition demanded that all names given to organisms become applicable only from 1758 hence, with the earlier names taking precedence over later names. In other words, the first name stands. Linnaeus provided a massive list of species in the 1758 publication. Some of the species he had collected himself, but many others came from the entourage of students who studied under him. Since Linnaeus named so many species, and his listing falls at the starting point of 1758, his genus and species names are still used today. Often the scientific literature will list the year the species was named along with the person who named it. An example is the northern pike, which may appear as Esox lucius Linnaeus 1758. If the genus has changed since the original naming, parentheses will surround the person's name in the listing. Linnaeus spent many years providing names for organisms, so it is common to find "Linnaeus" or simply "L." following scientific names.

The designation of a single, universal scientific name for each species was a giant step forward for zoologists and botanists. Scientists around the world used the same name to describe the same species. No matter how far apart they worked, scientists could finally exchange and share data with the confidence that all concerned were working with the same organism.

Both Linnaeus's organizational system and Cuvier's contribution of the "type" organism also proved to be great aids for zoologists and botanists trying to determine whether a newly found organism was actually a member of a new species. The taxonomical hierarchy provided a clear path to the correct group of comparison organisms, while the "type" provided one—and only one—accepted description for that species.

Linnaeus was also well known for his immense collection of plants and animals. Over his lifetime, he received specimens not only from his nearly 200 students but from botanists and zoologists worldwide. His work continued after his death most notably through his student Johan Christian Fabricius (1743-1808), a Danish zoologist. Fabricius began to classify the huge class of insects based on their mouthparts. His work encouraged early eighteenth-century entomologists to describe and arrange the invertebrates.

The Linnaean Society formed shortly after Linnaeus died, and became the caretaker of his library and specimen collection, which had become the largest in Europe. His manuscripts and collection are now housed in London.

Since Linnaeus developed his classification system, scientists have made new discoveries of organisms, and have proceeded with a wide range of morphological, genetic, and other studies. As new research presses on, the placement of plants and animals within the taxonomical hierarchy may change, but Linnaeus's classification system remains the foundation that directs the way scientists look at the natural world.

LESLIE A. MERTZ