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BENEDICT: NEW VARIETIES OF NEPHROLEPIS 209 
As preliminary to such description, it will be appropriate to 
consider some of the significant features of these variations, and 
to compare them with other variations of possibly similar nature. 
This discussion will be dealt with under two headings: viz., 
Earlier studies of orthogenesis; Conditions under which Nephro- 
lepis varieties have originated. 
2. Earlier studies of orthogenests 
The term ‘“ orthogenesis’’ was proposed by Haacke to char- 
acterize what the zodlogist, Eimer, had been calling “bestimmt 
gerichtete Entwicklung,” i.e. ‘‘definitely directed evolution”’ as 
translated by T. J. McCormack.* It appears that the botanist, 
Nageli, had expressed views similar to those of Eimer at a some- 
what earlier date. The evidence upon which these writers built 
their conclusions was systematic, and dealt with groups of living 
species showing close relationship. An orthogenetic series for 
Eimer was, for example, a group of related butterflies, the relation- 
ships between which was traced by gradations in the markings 
which varied from one extreme to another, but were connected by 
intervening forms. Evolution was postulated as progressive in 
one direction or the other. 
Later writers, such as Cope, Hyatt, Grabau, and Osborn, have 
considered orthogenesis in connection with groups of organisms 
in geologic series. Grabau, for example, has illustrated ortho- 
genesis in gastropods} by series of fossils from successive strata, 
which show gradual but definite progressive variation in the 
sculpture of the.shells. 
A third line of orthogenetic study is illustrated by Coulter’s 
recent discussion of orthogenesis in gymnosperms.} This, of 
course, is similar to that of Grabau, Hyatt, etc., in that it deals 
with extinct forms, but differs in being applied toa much longer 
period of variation and to less closely related forms, and it differs 
further in that it deals with the evolution of organs as such, the 
egg, proembryo, seed, etc., rather than with the phylogeny of 
organisms. eS 
* Eimer. On orthogenesis. Chicago. 1898. 
+ Studies in Gastropoda III. Am. Nat. 41: 607-651. 1907- 
t Science II. 42: 859-863. 1915. 
