“Thou shalt not wear a mingled stuff, wool and linen together.”
– Deuteronomy 22:11, JPS 1917 Tanach
“Seeing that the first two human beings who were born on earth were of different species, (Kayin and Hevel), one being the result of the evil genes of the serpent, the other that of Adam’s divinely inspired spirit, and we are commanded to keep our distance from the spirit of impurity, mixing the species has been forbidden for us as we have learned the fatal consequences which this could have.” – R. Bachya, commentary on Leviticus 19:19, sefaria.org
The fundamental differences between Kayin (Cain) and Hevel (Abel) are reflected in the nature of the offerings that each brought to H’Shem. “Cain brought of the fruit of the ground an offering unto the L-RD. And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the L-RD had respect unto Abel and to his offering; but unto Cain and to his offering He had not respect” (Genesis 4:3-5 JPS). A qualitative difference between Abel and Cain’s offerieng is inferred. Cain’s offering was linseed (Midrash Tanchuma, Bereishis 9), whereas Abel brought the choicest of his flock.
If Abel brought from his sheep, then this could correspond to the wool, mentioned in the previous commandment, while Cain’s offering would be represented by linen. The commandment forbids “wool and linen together.” This rendering would reinforce the underlying differences between Cain and Abel. If we are to be more like Abel, giving the best of ourselves as an offering to the L-RD through our good deeds, then, we should not compromise our standing with the L-RD by following the poor example of Cain at all. Rather, we should maintain excellency in all of our endeavors, both towards G-d and man.