66 STUDIES IN EVOLUTION 



furnishes another example, though in neither case has the 

 differentiation of structures proceeded far enough to result 

 in spines. The costate form (Planorbis costatus) 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, 50 shows a somewhat 

 further advancement. The species in the lowest beds (typus 

 Paludina Neumayri) are smooth and unornamented. Higher 

 in the strata they are angular and carinated, and at the top 

 of the series the shells are carinated, nodose, and sub-spinose 

 (typus Paludina Hoernesi). The living American genus 

 Tulotoma is closely related to the most differentiated species 

 (P. Hcernesi), and its approach to spinose features is more 

 pronounced. 



Under the phylogeny of spinose forms (pp. 23-25) an 

 outline of the life history of the brachiopod Atrypa reticularis 

 and derived species was presented. This being one of the 

 commonest types of Brachiopoda in the Silurian and Devo- 

 nian, 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 9 and others, that in the 

 Foraminifera, Crlobigerina 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 has also produced hyper- 

 trophy 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 maxi- 

 mum, and naturally with this a corresponding number of 

 normal variations. Some of these modifications, as spines, 



