Laudator Temporis Acti

Lucretius 5.916-924 (tr. Martin Ferguson Smith, with partial critical apparatus from Marcus Deufert's Teubner edition):

The fact is that, although there were manifold seeds of things in the ground at the time when the earth first produced animal life, this is no proof that beasts of mixed breed, combining limbs of different animals, could have been created. For the things that even now shoot in profusion from the earth—the various kinds of grasses and crops and exuberant trees—cannot, despite their abundance, be created intermixed: each proceeds in its own manner, and all preserve their distinguishing characteristics in conformity with an immutable law of nature.

nam quod multa fuere in terris semina rerum
tempore quo primum tellus animalia fudit,
nil tamen est signi mixtas potuisse creari
inter se pecudes compactaque membra animantum,
propterea quia quae de terris nunc quoque abundant        920
herbarum genera ac fruges arbustaque laeta
non tamen inter se possunt complexa creari,
sed res quaeque suo ritu procedit et omnes
foedere naturae certo discrimina servant.

923 sed res Goebel (1857) 26 (res sic iam Lambinus), comparans 2, 718–722 (cf. quoque 916; 1, 184–187) : sed si Ω : sed stirps Bockemüller : sed uis Lachmann

Monica R. Gale ad loc.:

As L. explains in book 2, the fertility of the earth is due to the abundance of different kinds of 'seeds' ( or particles) contained within it; but there are nevertheless limits on what it can produce, because not all types of atoms are susceptible of combination with all others (2.589-99, 700-29). This rule should apply to the distant past as to the present; and therefore the argument that hybrids might have come into existence because 'there were many seeds of things in the ground' falls.

See the list of parallels in Gordon Lindsay Campbell, Lucretius on Creation and Evolution: A Commentary on De Rerum Natura, Book Five, Lines 772-1104 (Oxford: Oxford University Press, 2003), p. 334:

Anaximander DK12 A30; ?Pythagoras DK58 C6; Empedocles DK31 B57, B59, B61, B62, B71; Anaxagoras DK59 A; (Diog. Laert. 2.9; Hippol. Ref. 1.8.12); Archelaus DK60 A4; Pl. Prt. 320c ff.; Genesis 1:21 and 2:19.