44 OUR NATIVE FERNS AND THEIR ALLIES. 



present aspect of a line of organic development, destined to 

 become something else in the future, as it was something else 

 in the past," — a definition in accord with the now universally 

 accepted biological doctrine respecting the origin of species. 

 Species among ferns are founded chiefly on differences in the 

 cutting of the fronds and their method of venation. 



1 1 7. Varieties. — Many forms differing only slightly from 

 the ordinary specific types, and yet capable of transmitting their 

 variations from generation to generation, are regarded as vari- 

 eties. It was the opinion of a prominent botanist, that all 

 so-called varieties among the lower plants " were purely the 

 result of the accident of environment, and never of cross-fertili- 

 zation." Since a species which varies in some minor particular 

 is likely to revert to the ordinary form as soon as the normal 

 conditions of soil, moisture, or environment are restored, there 

 is no scientific foundation for the multiplication of varieties to 

 serve as rubbish in works on systematic botany. There is a 

 tendency on the part of a few authors to multiply varieties in- 

 definitely, and of a single species as many as sixty-five varieties 

 have been described. The mania for naming new varieties is 

 quite universal, but is usually transient, and seldom affects one 

 a second time ; with some, however, it becomes chronic, when 

 more vigorous treatment is necessary. In the systematic por- 

 tion of this volume varieties that are deemed worthy to stand 

 as such are printed in the same bold-face type as the species. 

 Others less marked are noted in italics under the descriptions 

 of species. A true variety is doubtless the early stage of a 

 species in process of separation from its parent form. 



1 1 8. Genera. — The genera of ferns are founded mostly on 

 the arrangement of the sporangia on the veins, as well as the 

 character, shape, and position of the indusia. The generic 

 limits, however, are largely matters of opinion, and vary among 

 different authorities. 



1 1 9. Tribes. — Genera are collected into tribes, according 

 as they agree in the position and arrangement of the sporangia 

 in clusters or sori, or resemble each other in mode or habit of 

 growth. 



1 20. Sub-Orders. — Tribes are grouped into sub-orders 

 according as they agree in the characters of the sporangium, its 



