MIXED SPECIES (Heb. כִּלְאַיִם; kilayim), prohibition mentioned twice in the Bible. Leviticus 19:19 states: "Ye shall keep my statutes. Thou shalt not let thy cattle gender with a diverse kind; thou shalt not sow thy field with two kinds of seed; neither shall there come upon thee a garment of two kinds of stuff mingled together." Deuteronomy 22:9–11 states: "Thou shalt not sow thy vineyard with two kinds of seed; lest the fulness of the seed which thou hast sown be forfeited together with the increase of the vineyard. Thou shalt not plow with an ox and an ass together. Thou shalt not wear a mingled stuff, wool and linen together." From these two passages the sages deduced six types of mixing of species which are forbidden: the mixing of seeds; the grafting of different species of trees and vegetables; the mixing of seed in a vineyard; the hybridization of domestic and wild animals; plowing or driving with domestic or non-domestic animals of different species; and the mixing of wool and linen (*sha'atnez).
The prohibitions against mixing species are defined in Mishnah Kilayim 8:1 "It is forbidden to sow diverse kinds in a vineyard or to suffer them to grow, and it is forbidden to have any benefit from them. It is forbidden to sow diverse kinds of seed or to suffer them to grow, but they may be eaten and certainly benefit may be derived from them. Mixed materials are permitted for all purposes, only the weaving of them being forbidden. Hybrid cattle may be reared and maintained; it is forbidden only to breed them." The many halakhot connected with the laws of mixed species are taught in the Mishnah, Tosefta, and Jerusalem Talmud of the tractate *Kilayim. The chief problems relating to those laws are detailed below.
The Mixing of Seeds
The prohibition applies to the sowing together of two kinds of grains if they are regarded as belonging to different species (see below), or of grain and legume, as well as of other edible plants. A lenient ruling was given regarding vegetables, which were customarily sown in small beds, and it was permitted to sow five species at specified distances from one another in a bed one cubit square and with variations even 13 species (Kil. 3:l). According to most authorities, it is obligatory to separate fields sown with different species by the space of a rova (104 square cubits) or of three furrows (two cubits). In the opinion of some commentators, including Solomon Sirillio and Elijah Gaon of Vilna, the measures mentioned in the Mishnah (Kil. 2:6–10) refer to the size of the plot near which a different kind may be sown (and not to the space by which they must be separated), since plots of this size and larger have the appearance of separate fields, and there is no fear that they may be thought to have been planted indiscriminately, nor is there any risk that the different species will derive sustenance from one another. The prohibition of mixed seeds applies only in Ereẓ Israel, while the prohibitions of the other mixed species are of universal application (Kid. 39a).
The Mixing of Trees
This is not mentioned explicitly in the Bible but is inferred from the juxtaposition of verses (Lev. 19:19), "Thou shalt not let thy cattle gender with a diverse kind; thou shalt not sow thy field with two kinds of seed," which were interpreted to mean, "Just as the prohibition of cattle refers to mating, so does that of the field to grafting" (Kid. 39a), i.e., it is forbidden to graft two plant species in the same way as it is forbidden to mate two animal species. Some inferred the prohibition of grafting plants of different species from the beginning of the verse (Lev. 19:19): "Ye shall keep my statutes"; Sifra, Kedoshim (Perek 4:17) and the Jerusalem Talmud (Kil. 1:7, 27b) explain that the word ḥukkah ("statute"), is connected with the root ḥakok ("to carve"), i.e., that it is forbidden to change by grafting the original form "carved out" by the Creator at Creation. The prohibition applies to grafting a tree onto a tree, a vegetable onto a tree, and a tree onto a vegetable (Kil. 1:7). However, it is permitted to plant different trees side by side and to sow vegetables or grain among trees.
Mixing in the Vineyard
The laws of mixed species in the vineyard are stringent and complex, and almost half of the tractate Kilayim is devoted to them. The Bible (Deut. 22:9) rules that the resulting vines and seed become forfeit, and it is forbidden either to eat them or to benefit from them. The prohibition applies to grain but not to any trees among the vines. Concerning vegetables and other plants there are differences of opinion in the Mishnah and Talmud as to which are forbidden by biblical law and which permitted. A distance of four cubits must be allowed between a vineyard and any species forbidden to be sown there. In the case of a single vine, however, it suffices to leave a distance of three or six handbreadths (Kil. 6:1).
Mixing of Cattle
According to the Mishnah (Kil. 8:1) "they may be reared and maintained, and it is only forbidden to breed them." "To rear and maintain" means that different species of cattle may be reared together without the fear that they will crossbreed. Some explain it to mean that the product of crossbreeding (e.g., a mule) may be reared. This prohibition applies to domestic and wild animals and to birds (bk 5:7).
Plowing and Driving with Two Species
The Bible forbids only plowing with an ox and an ass. The rabbis, however, explained that "Scripture spoke what was customary," i.e., people were accustomed to plow with an ox or an ass, but the prohibition applies equally to plowing with any two other species and to riding, leading, and driving with them (Kil. 8:2).
Problems of Definition
In the discussion of the laws of mixed species the problem of defining like and unlike species arises. Although criteria for determining whether a plant or animal belongs to one species or another are laid down, an examination of the pairs enumerated in the Mishnah that do or do not constitute mixed species shows that there is no identity between the term "species" used in the law of mixed species and the term as applied by the modern system of botanical and zoological classification. Mixed species were determined by a tradition crystallized in the course of many generations (cf. Tosef., Kil. 1:3–4). Indeed two plants which are now classified as belonging to different species or even to different genera are reckoned as the same species for the law of mixed species (e.g., wheat and tares; Kil. 1:1). In contrast, however, different strains of the same species are regarded as different species (Kil. 1:6). With regard to mixed seeds an amora in the Jerusalem Talmud (Kil. 1:5, 27a) summarizes: "in some cases [the form of] the fruit is the determinant, and in others the leaf," while another amora notes: "in some cases the taste of the fruit is the determinant."
One of the assumptions in the prohibitions of mixed trees is the possibility of crossbreeding by grafting the scion of one species onto the stock of a second. Thus it is pointed out in the Jerusalem Talmud that grafting the almond onto the terebinth produces the *pistachio, a fruit similar to that of both these species but systematically very far removed from the almond. It is almost certain that a graft of such a nature will not take, and it is certain that a species which has the median characteristics of the scion and the stock cannot be obtained by grafting. The early scholars saw an analogy between the grafting of plants and the crossbreeding of animals, but this latter could be compared to the cross-pollination of plants, a technique unknown to the ancients.
These views on grafting stem from the once-accepted assumption that environmental factors were liable to change the hereditary characteristics of the creature (see *Biology). The opinion that new species could be created by grafting belongs to agricultural folklore, and also to Greco-Roman "science," and from there entered into rabbinic literature. Because of the prohibition of mixed species, Jews were unable to test the truth of this notion. Many halakhot on the subject commence with the formula: "If a gentile grafted" species A with species B, then species C is produced. It should be stressed, however, that halakhot of this nature, common in the Tosefta and the Jerusalem Talmud, were not incorporated in the Mishnah (see *Kilayim).
Reasons for the Precept of Mixed Species
Some of the reasons given for the prohibition stemmed from the above-mentioned belief that the effects of environmental factors are hereditary. To the same category belongs the reason for forbidding change in the order of Creation. Naḥmanides gives this reason in his biblical commentary (to Lev. 19: 19), adding that if the crossbreeding of a horse and an ass produces a mule, which is a miserable creature that cannot beget, so too when mixed species of trees are grafted, "their fruit does not grow thereafter." Maimonides (Guide 3, 49) explains that the man who couples creatures of different species defies the laws of nature and of ethics, and similarly in the grafting and mixing of plants. It was part of the false beliefs of idolators that this served as a specific for fertility (ibid. 3, 37). That cross-breeding was unnatural was an early belief: Josephus (Ant., 4:229) explained that "nature delighteth not in the conjunction of things dissimilar." Rabbenu Nathan, av ha-yeshivah (Ereẓ Israel in the 11th century), gives an agricultural reason, that one species prevents the development of the adjacent one (commentary to Mishnah Kil. ch. 1). A similar reason for the prohibition of mixed seeds in the vineyard was given earlier by Philo: "since as a result of it too great a burden is put upon the earth" (Spec. 4:211). Some Greek and Roman agricultural writers laid down that summer plants which impoverish the soil should not be sown in the vineyard (Pliny, Naturalis 18, 101) and that it is forbidden to sow intermediate plantings in a vineyard (Geoponica 5, 11).
As against those who sought to rationalize the prohibition, Rashi concluded: "These statutes are a royal decree, for which there is no reason." In point of fact it is impossible to determine the reasons for the prohibition. Post factum, however, it seems that, as a result of the care taken by Jews in this matter, the fields were kept free of weeds and the purity of plant species was preserved. It is also possible that it was a contributory factor to the success of Jewish agriculture in Ereẓ Israel.
In the present, prohibitions of mixed species have raised a number of problems for farmers who adhere to these laws. Thus they are prevented from sowing vetch with grain as fodder in order to prevent the vetch from trailing on the ground. The problem was solved by the introduction of strains of vetch which do not trail. In connection with the prohibition against grafting trees of different species, experiments have taken place on stocks belonging to the same species as the scion, but so far no satisfactory solution to the matter has been found.
Loew, Flora, 4 (1934), 291ff.; J. Feliks, Ha-Ḥakla'ut be-Ereẓ Yisrael bi-Tekufat ha-Midrash ve-ha-Talmud (1963); idem, Kilei Zera'im ve-Harkavah (1967).