OF PLANORBIS AT STEINHEIM. 17 



of well-marked old age metamorphoses in most of these large shells, is therefore another 

 sign of the innate strength of the progressive series. 



The progressive series are, therefore, not persistent but variable types, and consequently 

 we can consider them as possessing a capacity of adjusting themselves to the changes 

 in their environment, which affected the retrogressive series unfavorably. The reference 

 of these matters to the organization itself as a primary standard is farther sustained 

 by the behavior of each of these series. 



Thus each series is distinct from every other in the amount of change which it 

 exhibits, and in the partly retrogressive series we see the contention of two opposing 

 tendencies. The i-epresentative chai-acteristics, the sulcations and carinations, and increas- 

 ing involution of the whorl, are equally with the increase in size and spirality, marks of 

 strength. If so, the third sub-series exhibits most decidedly this battle of the tendencies, 

 and assuredly the first sub-series, where the size is temporarily increased, is a still stronger 

 instance of a similar kind. 



Here we appear to have a display of energy or force which probably did not arise in 

 the retrogressive species themselves, but was inherited from PI. levls, and we see it in 

 these races unsuccessfully resisting the deadly influences of the unfavorable environment. 

 These races, therefore, present in this respect, as well as in their forms, changes 

 which may be compared to those in the life of any individual of the progressive series, 

 which passes through a full series of changes or metamorphoses ; that is, one which has 

 not only a series of young and adult progressive changes, but also a series of retrogressive 

 or old age changes. 



If we analyze the phenomena presented by such an individual, we find, first, that it is 

 smooth, discoidal, and in a word similar to PL levis ; then that the whorl shows a flatten- 

 ing taking place above and below, with an increase in the amount of involution, and in 

 spirality ; then sulcations begin to appear, and longitudinal carinse, then as it passes its 

 adult condition, and is affected by disease or by old age, there is a tendency to suppress 

 the longitudinal carinations, and substitute more prominent costae or transverse lines of 

 growth, to decrease in size, and destroy the spiral. All of these last are changes attribut- 

 able to weakness in the organization of the animal. The prominence of the transverse 

 costae is due to longer periods of rest in building up the shell, and the consequent accumu- 

 lation of shelly secretions at intervals ; the decrease of the size in whorl self-evidently to 

 the same cause, and this also accounts well for the loss of symmetry in the spiral, which 

 can only be maintained by a constant increase in the building up of the shell. Here we 

 perceive the same contest of tendencies. There is the inherited strength of the consti- 

 tution building up the organization in size, and in all its progressive characteristics, and 

 resisting functional waste. There is then, in all outgrown specimens, though to an unap- 

 preciable degree in some, a retrogression, and in others a well marked retrogression, in 

 which the functional waste overbalances the supply of nutriment, and the organization 

 loses its progressive characteristics in a series of retrogressive changes. The contest is 

 decided at last as it must always be, in fiivor of function, the representative of physical 

 forces, which exhausts, conquers and kills by continuous friction. Thus, we can i-eadily 

 understand that each of these series, whether progressive or retrogressive, can so far as its 

 collective life is concerned, be compared in the closest manner with the life of an 



