Page:The New International Encyclopædia 1st ed. v. 04.djvu/929

* CLASSIFICATION OF ANIMALS. 81!> CLASSIFICATION OF ANIMALS. well for upward of fifty years. There is not a suspioion existing in the niiml of any one at all acquainted with the subject that the owner of either of them has deviated in any one instance from the pure blood of Mr. Bakcwell's tlock: and yet the difference between the shce|l possessed by these two gentlemen is so great that they have the appearance of being quite different va- rieties." Darwin's explanation of the lack of intergrades between many species differs from the foregoing, -and depends on the assumption that, of the descendants of any species, those that are most unlike are most apt to survive; so that where the number of forms (varieties) is verj- great, the intermediate ones (the inter- grades) are puslied to the wall and are extin- guished. In Darwin's words: "As in each fully stocked country natural selection necessarily acts l)y the selected form having some advantage in the struggle for life over other forms, there will be a constant tendency in the improved descendants of any one species to supplant and exterminate, in each stage of descent, their prede- cessors and their original progenitor. For it should be remembered that the competition will generally be most severe between those forms which are most nearly related to each other in habits, constitution, and structure. Hence all the intermediate forms between the earlier and later states — that is, between the less and more improved states of the same species — as well as the original parent species itself, will generally tend to become extinct. So it probably will be with many whole collateral lines of descent, which will be conquered by later and improved lines." This striking passage of Darwin deserves an experimental or statistical test. Until that is had. it can only stand as an excellent illustra- tion of Darwin's logical treatment of special difficulties in his theory. Another important attribute of a species is that its members are frequently sterile, with even closely related species. It would, however, be a mistake to assume that, on the one hand, all species, or even the hybrid offspring of crossed species, are always sterile. It is likewise an error to suppose that, on the other hand, sterility is confined to crosses between species. Breeders of animals are accustomed to find a considerable percentage of sterility between members of the same species ; and. when the members are close blood-relations, the percentage becomes very high. Nevertheless, the sterility of hybrids (crosses between species) is so common that Cuvier gave it as the sure criterion of species. For further facts and explanations of this question, see Hyuridity. Finally, species are adapted to the conditions in which they live. By many this characteristic is considered a fundamental one ; so that, when we have explained how their adaptations ari.se, we have explained how the species arose. It may be worth while, here, to state that there is a lack of unanimity in respect to the adaptive natire of all specific characters. Those who as- sert such adaptation insist that it cannot be denied that any given apparently useless organ may not be useful, or may not have been useful in some period of the animal's life, or that of its ancestors. See EvoLtT-noN ; X.^tirai, Seij:c- TION ; and similar titles, and the books men- tioned thereunder. Varietji, in natural history, is the designation of a group subordinate to species, distinguished from a race cliieliy by the circumstance that a race is artificial. Varieties are regarded as the first step in the formation of species. They are not to be confused with variations, since, like species, they are groups of similar individuals. The question of when to call two differing lots of animals species and when varieties is not capable of satisfactfiry solution, except by erect- ing an artificial and arliitrary bar separating them. In the absence of any such arbitrary rule, we find some naturalists 'splitting' one species into many, by elevating the varieties to specific rank; while another naturalist will 'lump' several species, reducing them to the rank of varieties of one species. Jlany authors would define varieties to be founded on differences due to environment — differences that will disappear with changing environment. Those wlio are radical in recognizing by name niinute and tran- sitory differences of this kind confess that iliey cannot recognize with certainty their own varieties, unless they know the haliitat of the specimen. Other authors regard the difference between variety and species to be solelj' one of degree of divergence in characters. As varieties became recognized, the custom was formed of adding a varietal name to the specific name, 33 has already been explained. Genus. — A category having a rank between species and family, and consisting of a group of species deemed to be more closely related to each other than to anything else. There is abso- lutely no criterion by which we can distinguish between a genus and a species, on the one hand, and a family on the other: but it is often a fact that, when a collection of related species is ar- ranged in an orderly fashion, more or less sharp breaks occur, which enable naturalists to draw lines. Consequently, the boundaries of genera are being constantly widened or restricted; and generic names change and disappear as a result of fusion of genera or of splitting up of a genus. Since the genus is that under which species are usually indexed, this instability of generic names is highly regrettable. What is needed is a rec- ognition of the subjectiveness of generic names and of the prime necessity of the stability of nomenclature: no changes in generic names should be made without the best of reasons. The name of the genus (always written with a capital initial) is combined with that of the species. The name of the cat genus, for ex- ample, is Felis; and Felis leo, Felis tigris, Felis catus are the names of the li<m, tiger, and wildcat species. These cats are more alike to one another than any one of them is to the dogs. The latter belong to the genus Canis. The size of a genus varies with the number of species it con- tains; some genera containing a hundred spe- cies or more, while others contain only one. Dar- win believed that the species of a large genus are more variable than those of a small one. In any case, the species wifliin a genus are not regarded as having diverged very widely from their common stock, and the criteria for generic distinctions are largely superficial characteris- tics. Famihi. — A group of genera having a certain resemblance to one another, which indicates common descent, or 'family connection.' Fea- tures of structure more or less external, and 1)C- longing to parts, as limbs, wings, teeth, horns.