BRYOZOA. 915 
Trepostomata.] 
In the Minnesota shales, the minor or generic types of structure especially were 
as yet most unstable, and the mixture that resulted in consequence is sometimes so 
great and perplexing that it is perhaps impossible to do full justice to the affinities 
of many species by*any known practical method of classification. 
The nearest approach probably to this desirable end is to be attained by the strictly 
genealogical system of classification, which I may confess I am more than inclined 
to adopt fully. The intricacy of this comparatively new and little understood system 
seems to be the chief bar to its early and complete acceptance by naturalists. And 
yet, so far as my experience goes, there is nothing very simple in the ramifications 
of organic differentiation. On the contrary, forms are so intertwined in their rela- 
tions that to unravel them is a matter of the utmost difficulty and patient inquiry. 
One of the more common of these difficulties is when we find a number of forms 
agreeing apparently closely in all characters assumed to be generic, and of which 
we have traced out the derivation of each so that we know them to have originated 
in different stocks or lines of development. Among many cases of this kind, that of 
Homotrypella ? ovata may serve as an example. This species has all the essential 
characters of Homotrypella, and yet I believe I can show conclusively that it repre- 
sents a departure from a line that originated in Homotrypa and later on, indeed soon, 
developed into the Hridotrypa mutabilis Ulrich, group of species. 
~ We know other species as well that stand in similarly equivocal relationship to 
Homotrypa. A careful study of these brings us to two conclusions: (1) Homotrypa 
generally manifested an inherent tendency to variation in that direction (7. e. to 
develop mesopores), and (2) that such forms as Homotrypa similis Foord, are to be 
regarded as reversions from the line of Homotrypa-Eridotrypa. Some of the questions 
involved would be more easily answered were it not for the almost contemporaneous 
existence of the variously differentiating types.* 
*My studies have served in a number of instances to throw light upon several as yet little developed thoughts in 
evolution. Chief among these is the one occasionally referred to by me as a “Tendency to variation in certain direc- 
tions.” This expression may sound simple enough, but the conditions expressed, providing they have been read aright, 
are really of great importance in the classification of animal nature. Of course I cannot here enter into a full discussion 
of the theory, but a few ideas and facts bearing upon it seem desirable. 
As results of presumed ‘‘tendencies”’ we find conditions that may be expressed as follows: After a species has once 
thrown off varieties of certain kinds, and these have died owt, you may expect similar variations from continued decend- 
ants of the type or species. OUases: (1) Dekayella prenuntia and varieties, and corresponding D. ulrichi and varieties (see 
remarks under description of Dekayella); (2) the lower and middle Trenton species of Callopora, ampla, pulchelia, persimilis 
and dumalis, described on succeeding pages, corresponding respectively to species subplana, dalei and ramosa, and an 
undescribed species, of the Cincinnati rocks; (3) the Lower Helderberg and Devonian species now classed as Thamniscus, 
though derived, like the more typical Carboniferous and Permian species of that genus, from Polypora, are not the direct 
ancestors of the latter, the first set of species having died out before the second were evolved; (4) Fenestella exhibited a 
continual tendency to throw off varieties and species that gradually assumed the characteristics of Polypora. Many other 
cases might be cited, but if those mentioned are followed up by the student I have no doubt he will find enough to con- 
vinee himself that tendencies in variation or evolution were preserved dormunt under retrogression, but manifested 
quickly enough when the proper conditions were presented. May not this idea explain the peculiar reapparition of 
cyclostomatous types discussed on pp. 121 and 122? 
Two other thoughts have suggested themselves in this connection. The first is that varieties and species were in 
some instances reabsorbed into the parent stem. The other relates to an approximation in structure (1) by contemporaneous 
forms or species that have had a common origin (e. g. Callopora undulata, C. incontroversa, C. angularis and C. ampla, all vary- 
ing toward C, multitabulata), and (2) by forms known to have been derived through different lines of development gradually 
assuming similar characters. as in the case of Homotrypa minnesotensis-Homotrypella ? ovata above described, 
