SPECIES. — A biological entity in which physiological isolation permits, on the one hand, the transmission to descendants of the complex of inherited traits from ancestors—traits common to the various phenotypes or races that compose it—and, on the other hand, prevents it from mixing with other closely related complexes. According to this description, a species does not mix with another, thereby distinguishing itself from the races that compose it, which can interbreed with one another. The species is a reality perceived by man. The author of Genesis states that God, in creating plants and animals, commanded the earth to bring forth the former and the waters to produce the latter, each according to its kind.
When studying lower organisms, the zoologist, although unable to grasp the intimate differences of each protoplasm—which are undoubtedly of a chemical nature—cannot doubt their existence when observing that each specific form deposits organic and even inorganic products, such as salts of differing composition, and that diversity is not limited to the elaboration of specific chemical substances but that each assumes a different physical aspect from species to species, or from one group of species to another. In bacteria, the species manifests itself in biological behavior; even the filterable viruses, which, given the current state of our knowledge, require living cells to multiply, demonstrate their specificity through the varied reactions they provoke in the animal or plant organism that hosts them. These facts seem to prove that life cannot exist without species and that, presumably, species arose from the very origin of life, in the sense that protoplasm or living substance did not organize itself uniformly but in specifically different ways from the beginning. The theory of the specific differentiation of protoplasm, subsequent to the emergence of life on Earth, does not seem sustainable when objectively examining the facts.
It is not enough for the notion of species to be based on more or less noticeable differences among organisms; these differences must separate groups of similar individuals—descended from a single individual through apogamy or parthenogenesis, or from a pair of individuals of different sexes through amphigamy—capable of transmitting their traits to their descendants. For a group of organisms sharing common traits to be designated as a "species," the hereditary transmission of these differences is therefore a necessary but not sufficient condition. A precursor of Linnaeus, John Ray, who lived in the second half of the 17th century, states in his *Historia plantarum* that the traits of a species perpetually retain their nature through parents or seeds and cannot be produced from seeds of another species. Linnaeus, as early as 1738, confirmed the perpetuity of the species, affirming that this entity was created by God: *species tot sunt quot diversas formas ab initio produxit infinitum Ens*. He thus established the principle that he himself later upheld in the 12th edition of his *Systema Naturae*, where he acknowledged the existence of plant varieties derived from accidental causes such as climate, soil, temperature, winds, etc., and limited the original creation to a few forms which, through interbreeding, would then produce the species existing today.
From Linnaeus to the present day, the concept of species has become increasingly precise, particularly regarding the conditions recognized as necessary for a group of similar organisms to be distinguished from other more or less related groups and considered a species.
A first difficulty arises from the inherent contradiction in the fact that specific traits are essentially hereditary yet can undergo modifications. The species is transmitted from generation to generation, but it is equally proven that intrinsic and extrinsic causes can induce modifications in descendants—sometimes fleeting, sometimes stable.
In evaluating these modifications, or even simply in assessing specific traits—which is to say, in the criteria for specific distinction—a conventional and subjective element comes into play, varying from one scholar to another depending on the nature of the organisms studied. It has already been noted that species react differently to environmental influences: now, the relationship between plants and their environment is so close that it cannot be assimilated to that between animals and their environment. The plant, so to speak, is bound to the soil and cannot escape the physical-chemical action of the terrain or climate, whereas the animal is entirely independent of the soil and its physical-chemical composition and can evade the effects of climate through migration and hibernation.
The assessment of the specific traits of animals cannot be conducted using uniform criteria across all groups, and it is easy to see that those adopted for distinguishing differences among insects cannot be the same as those used for distinguishing coelenterates or any class of vertebrates. For evident reasons of necessity, species are largely based on morphological traits, whether internal or external, which, however, vary not only from group to group but also from author to author.
From Buffon onward, a more general approach has adopted the physiological criterion of interfertility as a means of distinguishing specific traits, in the sense that all forms capable of producing viable offspring, even if morphologically different, belong to the same species, whereas those that are not fertile with one another or produce sterile hybrids constitute distinct species. This physiological criterion is rarely applicable because it requires the fulfillment of an experimental test that is very difficult to achieve, especially when dealing with animals. Moreover, between complete interfertility and sterility, there exists a series of intermediate conditions: 1) two forms that are interfertile in experimental conditions are not so in natural conditions due to: a) complete separation of their inhabited areas; b) lack of simultaneous maturation of germ cells; c) absence of mutual attraction or even repulsion; d) mechanical obstacles to mating or to the penetration of sperm into the egg. 2) Two forms that mate naturally or artificially produce hybrids that are fertile in one sex (homozygous) and sterile in the other (heterozygous). In these cases: a) germ cells are incapable of fertilization or being fertilized (in birds, small or even normal eggs that are not fertilizable); b) no germ cells are formed (in birds, complete interfecundity of the hybrid female); c) the sterility of the heterozygous sex disappears in the first backcross; d) the sterility of the same disappears in the second backcross; e) the sterility of the same disappears in the third backcross. 3) The above conditions may be accompanied in either sex by parasterility, meaning a statistically reduced production of mature germ cells and consequently few specimens reaching adulthood. 4) Hybrids are sterile in both sexes. 5) Viable hybrids belong only to the homozygous sex and are equally sterile.
All these conditions demonstrate that the definition of species based on the fertility and sterility of hybrids is not, by itself, as absolute as it might seem at first glance, and it is therefore advisable to qualify it by subordinately considering some non-physiological elements. It must nevertheless be affirmed that the entire systematics has been practically founded on morphological assessment.
Indeed, while it is easy to distinguish one species from another in the case of domestic animals and cultivated plants—whose numbers are extremely limited—it is equally difficult to distinguish species found in nature, because experience has been able to establish their identity in only a few cases. From this arises the necessity of admitting that alongside the few species that truly exist as such and are known to us, there exists a myriad of species that we consider as species in systematics because, based on differences in form or behavior, the naturalist presumes them to be real species, even though they may not be. These species may be designated as systematic species, in the sense that they are so considered within the system and without any certainty that they are truly distinct in reality. They are provisional species, in the botanist De Vries’s expression, pending future experience to confirm or deny their nature as species.
Given that knowledge of the real species—that is, the truly existing species—represents, in the vast majority of cases, an aspiration yet to be achieved, it can be affirmed that in various cases there is no coincidence at all between physiological affinity and morphological affinity.
The species was considered by Linnaeus as the real biological entity and was placed by him at the foundation of his *Systema Naturae* and his binomial nomenclature. According to this system, each species is designated by two names, inseparable under the specific aspect, since the second name, which refers to the species, has no value when dissociated from the first, which corresponds to the genus.
It should be noted, however, that while the species refers to an entity presumed to exist in reality, the genus is a subjective aggregate that expresses affinities among various species without any precise fact enabling us to define its true nature and extent.
At present, nomenclature is trinomial: the first name is always that of the genus, the second refers to the species, and the third to the subspecies or race. The genus is still usually an entity founded especially on morphological affinities, evaluated according to the subjective and personal criteria of each author, and generally corresponding to the Linnaean species; the species is the real entity, physiologically more or less separated from others by incomplete interfertility or cross-sterility; the subspecies or race, designated by the third name, is a substantially genotypic form linked to a determined geographical and ecological territory.
With the advent of experimental genetics, however, the concept of species has acquired an even more precise meaning. The specific characters are the extinction and separation of genes aligned in the chromosomes and that influence one another and, in turn, are influenced by one or more external factors, whence the appearance of phenotypes that are also the result of interactions between environmental factors and genetic factors. It is well to recall that many plants possess several phenotypes for flower color, depending on whether they bloom at medium, high, or low temperatures; that butterflies with two annual generations exhibit two seasonal color phenotypes, one in spring and another in summer-autumn, in relation to high or low temperature or, in tropical countries, in relation to the dry season or the rainy season.
Modern genetics has also demonstrated that many phenotypes to which specific character is usually attributed are the result of crosses, and that these, depending on whether they are in a homozygous (pure) or heterozygous (impure) state, are stable or unstable. In the first case, species may be considered real because they ensure their descendants, under uniform environmental conditions, the phenotypic continuity—that is, their outward appearance. Mendel’s laws, and especially the principle of the segregation of characters in hybrids and that of the independence of genes, explain, as is demonstrated by experience and statistical investigation, the mechanism of this phenomenon, which is moreover conditioned by territorial and geographical isolation. It is evident that the homozygous state of a phenotype resulting from hybridization cannot be determined or maintained where the possibility of further crosses with other related phenotypes persists. This is the case with cultivated plants grown separately and in isolation and with animals raised in cellular confinement. In nature, the most characteristic isolation is that offered by islands, especially if belonging to the same archipelago (Hawaii, Galápagos, Antilles, etc.). Examples of the effect of geographical isolation on the differentiation of forms are also provided by mountains and, respectively, by isolated valleys, deserts, etc.
There also exists ecological isolation, due to a different manner of reaction of the organism to distinct climatic environments, such as desert, forest, savanna, and to the different altitudes above sea level of such geological and vegetational formations.
These phenomena are particularly evident when the forms considered are interbreeding and can thus be considered as races of the same species. We are in the presence of a race whenever experience demonstrates that a population of phenotypes, animal or plant, is composed of individuals homozygous for characters that contrast with the corresponding alleles of another population of phenotypes more or less related to the one previously considered. A race may be defined as a population that maintains its own phenotype by reproducing in a homozygous state in a uniform environment. The expression “race” is substantially synonymous with that of subspecies, or elementary species, or Jordanons, from the French botanist Jordan, who considers as species groups of individuals that differ from one another by slightly accentuated characters. Races may also be geographical or ecological, depending on whether they are genotypes that are maintained in the homozygous state due to geographical isolation and are also called local races, or whether they are forms due to particular hereditary norms of reaction to the nature of the soil or climate. The origin of such races, both geographical and ecological, must be sought, at least in part, in new conditions determined by crossing, but for the most part in mutations: in both cases their stabilization is dependent on the intervention of isolating and selecting agents and chiefly on inbreeding. To understand what a mutation consists of in terms of the formation of a new race or subspecies and, possibly, of a species, it is always necessary to bear in mind what we have already said—that is, that the phenotype, namely the complex of somatic characters that individuate a homogeneous group of organisms, is due to a complex of elementary particles: the genes, aligned in the individual chromosomes. Experience has demonstrated that during the maturation of germ cells, external agents such as changes in temperature and humidity, chemical agents, radiation, can cause the loss or displacement of one or more specific genes in a single chromosome, or changes in the structure of one or more chromosomes, and finally changes in the general arrangement of the chromosomal complement. These gene, chromosome, and genome mutations are responsible for stable changes in the genotype and consequently in the phenotype. These are usually, however, small species, those that we have indicated as elementary species or Jordanons, arising within the sphere of a single collective or Linnaean species, this expression being used to refer to the species established by Linnaeus, large collective species that usually include
Biologists have also sought to determine whether other criteria, beyond those already discussed, can be used to identify species, but the results have been negative. Cytological criteria, especially those concerning the number and shape of chromosomes, have not yielded positive results, since often the number is the same in very different groups of animals, or is shared by all species within the same family (e.g., grasshoppers); in other cases, races of the same species have double, triple, or quadruple the number of chromosomes found in the original race. Chemical and serological criteria are also more useful for distinguishing races and lineages within a race than for distinguishing species from one another. Notable serological affinities have been observed between some monkeys and humans, but no one considers these forms to belong to the same species.
From what has been presented, it is clear that the definition of species we have given is the one that best corresponds to our current knowledge of the subject. However, the difficulties involved in experimentally verifying interbreeding or sterility make this definition more theoretical than practical.
Given the enormous number of known species and the need for systematic classification, zoologists are compelled not to rely solely on the difficult and often impossible physiological experimentation, but to consider other morphological, geographical, ecological, and behavioral traits as well.
SPECIE (philosophy): V. PREDICABILI.