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PRINCIPLES OF CLASSIFICATION
14. Classification.—It is plain that natural relations of some sort exist among living organisms. A dog is more like a cat than it is like a sheep. A dog is more like a sheep than either is like a butterfly. The very existence of such terms as animals and plants, insects and fishes, implies various grades of relationship. Classification is the process of reducing our knowledge of these grades of likeness and unlikeness to a system. By bringing together those which are alike, and separating those which are unlike, we find that these rest on fixed and inevitable laws. Classification is thus defined as "the rational, lawful disposition of observed facts."
15. Homology.—All rational classification of plants or animals concerns itself with homologies. Homology means fundamental identity of structure, as distinguished from analogy, which means incidental resemblance in form or function. Thus the arm of a man is homologous with the fore leg of a dog, because in either we can trace throughout deep-seated resemblances or homologies with the other. In every bone, muscle, vein, or nerve the one corresponds closely with the other. The "limb " of a tree, the "arm" of a starfish, or the fore leg of a grasshopper shows no such correspondence. In a natural classification, or one founded on fact, those organisms showing closest homologies are placed together. An artificial classification is one based on analogies. Such a classification would place together a cricket, a frog, and a kangaroo, because they all jump; or a bird, a bat, and a butterfly, because they all have wings and can fly, although the different kinds of wings are made in very unlike fashion.
16. Natural classification based on homology.—The closest homologies are shown by those animals which have sprung from a common stock. The basis of natural classification, which is an expression of the ancestry of blood relationship of animals, is therefore homology. So far as we know, the actual presence of homologies among animals implies their common descent from some stock possessing the same characters. The close resemblance or homology among the different races of men indicates that all men originally
ternal characters, having more to do with surroundings, are much more rapidly altered in response to demands of the environment.
A perfect classification would indicate the line of descent of each member of the series, those now living
came from one stock. As homology implies blood-relationship, so, on the other hand, common descent implies homology, the similar parts being derived from a common ancestral stock. It is sometimes said that the inside of an animal tells what it is, the outside where it has been. In the internal structure, ancestral traits are perpetuated with little change through long periods. The exhaving sprung in natural sequence, by slow processes of change, from creatures of earlier geological periods. It is said that in classification we have " three ancestral documents ": Morphology, Embryology, and Paleontology. In Morphology we compare one form with another, thus
Fig. 3.—Homology of digits of four odd-toed mammals, showing gradual reduction in number and consolidation of bones above.—After Komanes.
tracing resemblances and differences. In Embryology we trace the development of individuals from the egg, thus finding clues in heredity that will enable us to trace the development of the race. In Paleontology we study the extinct forms directly, thus often finding evidence as to the origin of forms now existing.
17. Scientific names.—Each of the different kinds of animal or plant is called a species. There is no better definition of species. Thus the red squirrel is a kind or species of squirrel, the gray squirrel is another, the fox squirrel a third. The black squirrel of the East is not a species, because black squirrels and gray squirrels are sometimes found in the same nest, born from the same parents.
A genus is a group of closely related species—one or more—separated from other genera by tangible structural characters. Thus all the squirrels named above constitute
Fig. 4.-Locusts (Schistocerca) taken on the Galapagos Islands, Pacific Ocean ; all descended from a common ancestor, but now scattered over
the various islands, and varying in size and markings. a, Schistocerca melanocera Stol (Charles Island); 6, S. intermedia borealis Snodgrass
a single genus. Other squirrel-like animals, as the chipmunk, the flying squirrel, the prairie gopher, or the prairie dog, belong to as many different genera.
In the binomial system, invented by Linnaeus and applied by him to animals in 1758, the scientific name of an animal consists of two words—the name of its genus and species taken together. The name of the genus comes first. It is a noun, in Latin form, though usually of Greek derivation—thus Sciurus, the squirrel, in Greek meaning shadow-tail. The name of the species is an adjective in meaning, placed after the noun and agreeing with it. Thus Sciurus hudsonicus is the name of the red squirrel, Sciurus carolinensis of the Eastern gray squirrel, and Sciurus ludovicianus of the fox squirrel; Sciuropterus volans is the flying squirrel, Tamias striatus the Eastern chipmunk, and Spermophilus franklini one of the prairie gophers. The specific name is usually a descriptive adjective—often the name of a locality, sometimes the name of a man. The authority usually written after the name of an animal is that of the one who gave it its specific name —thus Sciurus hudsonicus Erxleben, which means that Erxleben first called it hudsonicus. Usually the name of the authority is that of the discoverer of the species. When several names are given to the same animal they are called synonyms. The earliest of these names is the right name. All the rest are wrong.
18. Families of animals.—A group of related genera is called a family. The name of a family is derived from that of its principal genus, with the termination idee. Thus all the squirrel-like animals belong to the family of Sciuridm. All the sorts of mice to the Muridce, from the principal genus, Mus, the mouse. The rabbits are Leporidm, from Lepus, the rabbit, and the beavers Castoridm, from Castor, the beaver.
19. Higher groups of animals.—In the higher groups we first trace out the different plans of structure. There is