The Barddas of Iolo Morganwg, Vol. I., ed. by J. Williams Ab Ithel, , at sacred-texts.com
To consociate with evil will make one the lowest and meanest of all animated beings; therefore a wicked man,
p. 244 p. 245
when he dies, and his soul enters the meanest worm 1 in existence, becomes better, and ascends in the migration of Abred. From this has arisen the saying,--"Trample not on thy better," addressed to one who tramples on a worm voluntarily, and without a cause.
245:1 p. 244 "Pryf," a worm, originally written prif, probably obtained its name from its being the first prif = primus--link in the chain of Abred.
There seems to be some allusion to the doctrine enunciated here in those lines by Casnodyn, A.D. 1290-1340;
Thou didst prepare the slough of hell suitable for Satan;
The habitation for worms, where they will be in mutual strife.