The present work has grown out of a monograph entitled Myxomycetes of Eastern Iowa, published by the present author about eight years ago. The original work was intended chiefly for the use of the author's own pupils; but interest in the subject proved much wider than had been supposed, and a rather large edition of that little work was speedily exhausted. At that time literature on the subject in question—literature accessible to English readers—was scant indeed. Cooke's translation of Rostafinski, in so far as concerned the species of Great Britain, was practically all there was to be consulted in English.

In 1892 appeared in London Massee's Monograph of the Myxogastres, and two years later in the same world's centre the trustees of the British Museum brought out Lister's Mycetozoa. Although these two English works both claim revision of the entire group under discussion, the latter paying special attention to American forms, nevertheless there still seems place for a less pretentious volume which for American students shall present succinct descriptions of North American species only. The material basis of the present work consists of collections now in the herbarium of the State University of Iowa. In accumulating the material the author has had the generous assistance of botanists in all parts of the country, from Alaska to Panama, and the geographical distribution is in most cases authenticated by specimens from the localities named. The descriptions, in case of species represented in Europe, are based upon those of European authors; for forms first described in this country, the original descriptions have been consulted. A bibliography follows this preface.

In reference to the omnipresent vexed question of nomenclature, a word is perhaps necessary. De Candolle's rule, "The first authentic specific name published under the genus in which the species now stands," may be true philosophy, but it is certainly an open question how that rule shall be applied. If an author recognized and defined a given species in times past, and, in accordance with views then held, assigned the species to a particular genus, common honesty, it would seem, would require that his work be recognized. To assume that any later writer who may choose to set to familiar genera limits unknown before shall thereby be empowered to write all species so displaced his own, as if, forsooth, now for the first time in the history of science published or described, is not only absolutely and inexcusably misleading, but actually increases by just so much the amount of dÉbris with which the taxonomy of the subject is already cumbered.

In face of a work so painstaking and voluminous as that of Rostafinski, and in view of the almost universal confusion that preceded him, it would seem idle to change for reasons purely technical the nomenclature which the Polish author has established. Especially is this true in the case of organisms so very perishable and fragile as those now in question where comparative revision is apt to result in uncertainty. We had preferred to leave the Rostafinskian, i. e. the heretofore current nomenclature, untouched; but since other writers have preferred to do otherwise, we are compelled to recognize the resultant confusion.

Slime-moulds have long attracted the attention of the student of nature. For nearly two hundred years they find place more or less definite in botanical literature. Micheli, 1729, figures a number of them, some so accurately that the identity of the species is hardly to be questioned. Other early writers are Buxbaum and Dillenius. But the great names before Rostafinski are Schrader, Persoon, and Fries. Schrader's judgment was especially clear. In his Nova Genera, 1797, he recognizes plainly the difference between slime-moulds and everything else that passed by the name of fungus, and proposed that they should be set off in a family by themselves,[2] but he suggested no definite name. Nees (C. G.) also made the same observation in 1817, and proposed the name Ærogastres; but he cites as type of his Ærogastres, Eurotium, and includes so many fungi, that it seems unsafe now to approve his nomenclature. Schrader also has left an excellent account of the cribrarias, the basis of all that has since been attempted in that genus.

Persoon, in his Synopsis, 1801, attempts a review of all the fungi known up to that time. His notes and synonymy are invaluable, enabling us to understand the references of many of the earlier authors where these had otherwise been indefinite if not unintelligible. He makes a great many changes in nomenclature, and excuses himself on the ground that he follows, in this particular, illustrious examples! Unfortunately, so do we all!

Fries, in his Systema Mycologicum, 1829, summed up in most wonderful way the work of all his predecessors and the mycologic science of his time. In reading Fries the modern student hardly knows which most to admire, the author's far-reaching, patient research, the singular acumen of his taxonomic instinct, the graceful exactness of the Latin in which his conclusions are expressed, or the delicate courtesy with which he touches the work even the most primitive, of those his predecessors or contemporaries. Nevertheless in our particular group even the determinations of Fries are not conclusive. He himself often confesses as much. The microscopic technique of that day did not yield the data needful for minute comparison among these most delicate forms.

It remained for DeBary and Rostafinski to introduce a new factor into the description of species, and by spore-measurement and the delineation of microscopic detail to supply an element of definiteness which has no parallel in the work of any earlier student of this group. Under these conditions the revision undertaken by Rostafinski was of a most heroic sort. His work was almost a new beginning; and while in nomenclature he was inclined to follow the Paris Code, yet the inadequacy of the earlier descriptions often made such a course impracticable. The synonymy of Rostafinski is largely that of Fries, and upon this the Polish author attempts to apply the law of priority. In the historical note, wzmianka historyczna, accompanying the description of each specific form, he generally states the reason for the nomenclature he adopts, whether selected from the mass of supposed synonymy or introduced by himself de novo. Unfortunately, Rostafinski is sometimes purely arbitrary in his selections. He sometimes changes a specific or even generic name, otherwise correctly applied, simply because in primary etymological significance the name seems to him inappropriate. In such cases it is proper to restore the earlier name. Nevertheless Rostafinski is still our most trustworthy guide.

Of course, where later investigations have served to obliterate the once-thought patent distinctions between supposed genera or species, it is proper to unite such forms under the older determinable titles and this we have attempted. But wherever in the present work a name has been changed, the name of the earlier author will be found in parenthesis, followed immediately by that of him who made the change, and in general, recent practice, especially as expressed in the rules of the various codes, has determined the puzzling questions of nomenclature.

In justification of the use of Myxomycetes as a general title it may be said that in this case prevalent usage is not inconsistent with a rational application of the rules of priority. The Friesian designation Myxogastres was applied by its author in 1829 to the endosporous slime-moulds as a section of gasteromycetous fungi. Four years later Link, perceiving more clearly the absolute distinctness of the group, substituted the name Myxomycetes. In the same year Wallroth adopted the same designation, but strangely confused the limitations of the group he named. Wallroth seems to have thought Myxomycetes a synonym for Gasteromycetes Fries. In 1858 DeBary applied the title Mycetozoa to a group which included the then lately discovered Acrasieae with the true slime-moulds, both endosporous and exosporous. For all except the Acrasieae DeBary retained the old appellation, Myxomycetes. Rostafinski adopted DeBary's general name, but changed its application. As it has been shown, since DeBary's time, that the Acrasieae[3] have no true plasmodium, and are therefore not properly, or at least not necessarily, associated with the slime-moulds, there appears no necessity for the term Mycetozoa, and the question lies between Myxogastres and Myxomycetes. Of these two names the former, as we have seen, has undoubted priority, but only as applied to the endosporous species. The same thing was true of Link's designation until DeBary redefined it, but having been taken up by DeBary, redefined and correctly applied, Myxomycetes (Link) DeBary must remain the undisputed title for all true slime-moulds, endosporous and exosporous alike.

In arranging the larger divisions of the group the scheme of Rostafinski has been somewhat modified in order to give expression to what the present author deems a more natural sequence of species. The highest expression of myxomycetan fructification is doubtless the isolated sporangium with its capillitium. This is reached by successive differentiations from the simple plasmodium. The Æthalium may be esteemed in some instances a case of degeneration, in others of arrested development. In any event in the present arrangement, Æthalioid forms are first disposed of, leaving the sporangiate species to follow from plasmodiocarpous as directly as may be.

The artificial keys herewith presented proceed on the same plan and are to be taken, as such keys always are, not as definitive in any case, but simply as an aid to help the student more speedily to reach a probably satisfactory description.