Aristotle thought some fish were spontaneously generated and others not (also all eels and Testacea). "This occurs in ponds, especially one near Knidos, which,

Aristotle thought some fish were spontaneously generated and others not (also all eels and Testacea). "This occurs in ponds, especially one near Knidos, which, so it is said, on one occasion dried up at the time of the Dogstar and all the mud was taken out; water began to collect in it as the first <rains> came, and at that point tiny fishes appeared, as soon as the water began. This fish was a kind of grey mullet which is not produced as the result of copulation, and its size was about that of a small sprat: none of them had either egg or milt. (...) This evidence shows that certain fishes are produced spontaneously, and do not come out of eggs or from copulation" (HA VI.15 569a13-19, 25-26). This is something of a problem for Aristotle, since the formal cause appears to be absent in spontaneous generation. He likely would have been happier to discover that spontaneous generation does not, in fact, occur for animals. But he felt that the evidence forced him to concede that it does.