The ordinary irregular flowers may possibly be degenerated descendants of a more completely organized ancestor, and some of the cases of peloria may therefore be instances of reversion; some ancient _Linaria_ may, perhaps, have had all its petals spur-shaped, and the cases of irregular peloria now found may be reversions to that original form. When both regular and irregular forms of peloria occur on the same plant, as they frequently do in _Linaria_, the one may be perhaps considered as a reversion to a very early condition, the other to a later state, when all the petals were irregularly formed. But before we can assert the truth of this surmise we must have better evidence as to what the original condition really was than we have at present. The proximate cause of irregular peloria has been considered to be excess of nourishment, but evidence as to this point is very conflicting. Willdenow states that "radices peloriae, solo sterili plantatae, degenerant in Linariam," ('Sp. Plant.,' iii, p. 254); but this opinion is counterbalanced by that of others, while the frequent existence of both forms on the same plant, at the same time, seems to negative the supposition of any direct effect from external circumstances. The following are the plants in which irregular peloria has been most often observed: Aconitum Napellus. Delphinium elatum! Corydalis tuberosa. *Viola odorata! hirta. Impatiens Balsamina. Clitoria Ternatea. Cytisus Laburnum! Trifolium repens! Lupinus polyphyllus! *Gloxinia, var. cult.! *Linaria vulgaris! spuria. Elatine. triphylla. aeruginea. triornithophora. pilosa. chalepensis. cymbalaria! purpurea! decumbens. Pelisseriana. origanifolia. Digitalis orientalis. * purpurea! Calceolaria crenatiflora. rugosa. * var. cult.! Chelone barbata. *Antirrhinum majus! Rhinanthus crista galli. Pedicularis sylvatica. Pedicularis euphrasioides. Scrophularia aquatica! Sesamum indicum. Lamium. Mentha. Sideritis. Nepeta diffusa. Galeopsis Ladanum. Tetrahit. Galeobdolon luteum. Teucrium campanulatum! Plectranthus fruticosus. Cleonia lusitanica. Dracocephalum austriacum. Phlomis fruticosa! Vitex incisa. Aristolochia, sp.! Ophrys aranifera! Orchis simia. pyramidalis! latifolia! morio! papilionacea. mascula. latiflora. conopsea. Habenaria bifolia. Corallorhiza innata. Aceras anthropophora. Cattleya Moasiae! Phalaenopsis equestris! Pogonia ophioglossoides! The literature of peloria is very extensive. The following are the principal papers, not already mentioned, which relate to the subject, arranged under the genera, placing those first which are most subject to this anomaly (see also Regular Peloria).