Translated by Ralph Anzarouth and an anonymous friend
The fourth category is natural love, which is beyond the power of man's will to control and it is his nature which compels him and inclines him to love what resembles him either in substance, form or deed: [the object of his love] and his conduct will find favor in his eyes, whether they are good or bad. He will continue loving even without receiving any pleasure or benefit. The wonder of this is seen in the animal realm where each species will like its own kind without possessing intelligence or knowledge. And every raven will settle with its own kind without receiving any benefit at all.
Similarly, practitioners of the same craft will befriend each other, in spite of the mutual rivalry and aversion between fellow craftsmen. Since each person places his own interests before those of his fellow man, therefore, due to his jealousy and fear of being damaged by him, he will dislike him.
Plato said: “If you want to recognize somebody who is like you, look for somebody who befriends you without obvious motives”. Our Sages of blessed memory said (Talmud Bavli, treatise Bava Kama 92b): “Why does the starling mix with the raven, if not because it is of the same species1?”
This sums up the whole matter.
Note of the translators:
 This is Rabbi Eliezer’s interpretation of Leviticus 11, 15, whereas the Sages disagree and think that the starling belongs to a pure species.
The whole book Tzemach Tzadik in Hebrew (printed in Rashi characters) can be downloaded in PDF format and read online at Hebrewbooks.org. Chapter 5 about natural love is found at page 14.