Prohibited to Alter Nature

 Ramban (Vayikra 19:19) YE SHALL KEEP ‘MY STATUTES. “And these are they: thou shalt not let thy cattle gender with a diverse kind, etc. Statutes are the decrees of the King for which there is no reason given.” This is Rashi’s language. But our Rabbis have not mentioned that the reasons for the commandments mentioned in this verse are hidden from us, and that the evil inclination and the idolaters raise objections against them, except in the case of the prohibition against wearing a garment made of wool and linen, but not in the case of mating of animals of diverse kinds for which there is a reason, as will be explained later on. And the intention of the Rabbis in defining “statutes” as the laws of the King for which there is no reason was not that these are decrees of the King of kings for which there are no reasons whatever, for every word of G-d is tried. They meant only that “statutes” are like the enactments which a king promulgates for his kingdom, without revealing their benefits to the people, and the people, not sensing these reasons, entertain questions about them in their hearts but they accept them nonetheless out of fear of the government. Similarly, “the statutes” of G-d are His secrets in the Torah, which the people by means of their thinking do not grasp as they do in the case of ordinances” — laws which conform to the human conception of justice], but yet they all have a proper reason and perfect benefit. Now the reason for the prohibitions against kilayim [mixed kinds], is that G-d has created in the world various species among all living things, both plants and moving creatures, and He gave them a power of reproduction enabling them to exist forever as long, as He will desire the existence of the world, and He further endowed them with a power to bring forth only after their kind, and that they should never be changed, as it is said with reference to all of them at the time of Creation, after its kind. This driving force in the normal mating of animals is for the sake of preserving the species, even as human beings engage in sexual activity for the sake of having children. Thus one who combines two different species, thereby changes and defies the work of Creation, as if he is thinking that G-d has not completely perfected the world and he desires to help along in the creation of the world by adding to it new kinds of creatures. Moreover, the mating of diverse species of animals does not produce offspring, and even in the case of those that are by nature close to each other, from which offspring are born, such as mules, their seed is cut off, for they themselves cannot produce offspring. Thus from the point of view of these two matters the act of combining different species is despicable and futile. Even when diverse species of vegetation are grafted together, their fruits do not reproduce afterwards, and they too are prohibited because of the two above-mentioned reasons. This is the meaning of the prohibition, thou shalt not sow thy field with two kinds of seed, which in the opinion of our Rabbis constitutes a prohibition against grafting  But He has prohibited also the mere sowing together of diverse kinds of seed, because their nature and form change when they derive nutrition from each other, and thus each kernel of it is as if it were grafted together from two kinds. Similarly, He has forbidden to plow with an ox and an ass together, because it is customary among tillers of the soil to bring their working animals into one cattle-shed, and there they might come to mate with a diverse kind. And one of our colleagues adds to the reason for the prohibition against mixing seeds, that it is in order not to throw into disorder the primary forces which bring about the growth of the plants, when they derive nutrition from each other, as is indicated in the saying of our Rabbis in Bereshith Rabbah: “Said Rabbi Simon: There is not a single kind of herb that does not have a constellation in heaven which smites it and says to it, ‘Grow.’ Now he Thus he who mixes different kinds of seeds, denies and throws into disorder the work of Creation.