114 THE PERMO-CARBONIFEROUS RED BEDS OF 



Dybowsky. In contrast to this, it may be mentioned that but 4 fresh-water species 

 have been discovered in the whole of Norway. In Lake Baikal, all the depths ex- 

 plored (to 1,373 meters) have furnished species. Those living near the surface are 

 vividly colored, yet apparently make no attempts at concealment. Many of the 

 species are also highly spinous, though not sufficiently armed to be protected from 

 the fish. As these Crustaceans are voracious creatures, the spinous character has 

 probably been favored by the agency of cannibalistic selection. The lake has a 

 number of species of fish for which the Gammaridse furnish excellent food, but the 

 presence of a species of seal, predaceous fish, as well as the native fisherman, keeps 

 the fish below the danger point, thus allowing the Gammaridas to become very 

 abundant. 



"Similarly, in Lake Titicaca there is a wonderful specific development of a 

 kindred Crustacean, Allorchestes. One of the most spinous species (armatus) is 

 also the commonest, and, according to Faxon, occurs in countless numbers. 



"Packard shows that among certain moths, the caterpillars, as soon as they 

 aquired arboreal habits, met with favorable conditions in respect to food, tempera- 

 ture, etc., and that as spines and tubercles arose by normal variation, such features 

 being found useful for protection, were therefore preserved and augmented. 



" Che differentiation of Achaiinella has already been discussed as affording a 

 striking instance of free variation among the MoUusca. The evolution of the 

 Tertiary species of Planorbis at Steinheim, as described by Hyatt, furnishes another 

 example, though in neither case has the differentiation of structures proceeded far 

 enough to result in spines. The costate form (Planorbis co status) was tending 

 toward that end, but did not attain it. 



"The series of Slavonian Paludina, in the Lower Pliocene, as elucidated by 

 Neumayr and Paul, show a somewhat further advancement. The species in the 

 lowest beds (typus Paludina newmayri) are smooth and unomamented. Higher in 

 the strata they are angular and carinated, and at the top of the series the shells are 

 carinated, nodose, and subspinose (typus Paludina hoernesi). The living American 

 genus Tulotoma is closely related to the most differentiated species (P. hoernesi), 

 and its approach to spinose features is more pronounced. 



' ' Under the phylogeny of spinose forms (pp. 1 8 and 1 9 of Beecher's paper) an out- 

 line of the life history of the brachiopod Atrypa retiatlaris and derived species was 

 presented. This being one of the commonest types of brachiopods in the Silurian 

 and Devonian, often forming beds of considerable extent, it seems quite likely that 

 its prolonged development under favorable conditions for multiplication must have 

 had an effect on the amount and kind of variation. 



' ' It has been noticed by Brady and others that in the Foraminifera, Glohigerina 

 bulloides, Orbulina universa, etc., the pelagic forms comprise two varieties which are 

 generally distinct, a spinous form, and another with small, minutely granular shells. 

 The bottom specimens of the same species are also commonly without spines, and 

 often smaller. The interpretation seems to be that the large specimens indicate an 

 abundance of nutrition, which also produced hypertrophy of the normal granules 

 into spines. Some bottom specimens are large, but they are usually abnormal and 

 of a monstrous or pathologic nature. 



"From the foregoing examples, the conclusion to be drawn is that, with full 

 nutrition, there comes a numerical maximum, and naturally with this a correspond- 

 ing number of normal variations. Some of these modifications, as spines, have 

 arisen by hypertrophy. After having thus originated by growth force, they may 

 or may not be of use for offense, defense, or concealment, or in any way give their 

 possessor a distinct advantage." 



