Trepostomata.] MYO^OA. ^l5 



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 

 greg,t 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 diflferentiation. 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 

 Hdmotrypella ? 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 Eridotrypa 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 {i. 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 <3t 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, hut the conditions expressed, providing they have heen read aright, 

 are really of great importance in the classification of animal nature. Of coarse 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 lias once 

 thrown oft varieties of certain kinds, and these have died out, you may expect similar variations from continued decend- 

 ants of the type or species. Oases: (1) Dekayella prmnuntia and varieties, and corresponding D. ulricM and varieties (see 

 remarks under description of Dekayella); (2) the lower and middle Trenton species ot Callopora, ampla, puWiella, persimilis 

 and dumalis ' described on succeeding pages, corresponding respectively to species mbplana, dalei and ramosa, and an 

 undescribed' species, of the Cincinnati rocks; (3) the Lower Helderberg and Devonian species now classed as Thamrmcus, 

 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) Fen«st6Jto exhibited a 

 continual tendency to throw oft varieties and species that gradually assumed the characteristics of Polypora. Many other 

 oases might be cited, but if those mentioned are followed up by the student I have no doubt he will find enough to con- 

 h self that tendencies in variation, or evolution were preserved dormant under retrogression, but manifested 

 '^uickly^nough when the proper conditions were presented. May not this idea explain the peculiar reapparition of 

 cvclostoraatous 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 



• t ces reabsorbed into the parent stem. The other relates to an approximation in structure (1) by contemporaneous 



some ins an have had a common origin (e. g. CaUopora undulata, C. ineontroversa, C. anffularis and C. (Mnpla, all vary- 



forms °^ ^P®" .„^^^|^^^aJ)l^lata), and (2) by forms known to have been derived through different lines of development gradually 



ssumrng similar characters, as in the case ot Homotrypa minneeotcnsis-HomotrypelUi t ovata above described. 



