26 HYATT ON THE TERTIARY SPECIES 



reliable basis of estimating relative health of individuals, and, other things being 

 equal, the favorable or unfavorable character of their surroundings. These facts and the 

 peculiar reversions of PI. trochiformis in some of its characteristics, appear therefore to 

 be perfectly well accounted for by Darwin's theory of natural selection. In no other 

 way can we possibly account for the selection of but four out of the varieties of 1*1. 

 levis, and the continuous propagation and increasing intensity of the differences which 

 they exhibit. An examination also of the figures of the nearest allied forms of PI. levis, 

 such as PL Steinlieimensis , will show any one how many variations are lost in each form 

 or species of the series, and how few are contmued. This can only be accounted for 

 upon the supposition that those which survived possessed in some way advantages indi- 

 cated by their peculiar variations, which enabled them to propagate those variations, and 

 suppressed their less fortunate neighbors. 



It is impossible to imagine any uniform inorganic cause acting upon the comparatively 

 constant organization of PI. levis, and producing therefrom a number of radiating series, 

 genetically distinct from each other, and with all the other marks of independent consti- 

 tutional and hereditary traits above cited, unless we allow with Darwin a basis of original, 

 constitutional differences indicated by the tendency of PL levis to vary. Then it is easy 

 to see how the physical environment would cause a selection to take place in the differ- 

 ential characteristics of the varieties, and how the same causes continued, would perma- 

 nently fix the favorable differences in the race or series, causing it even to increase in 

 intensity until another change, as in PL tmwormisi might cause selective alteration of the 

 original differences, or even a reversion of these. 



All of these considerations when taken together appear to show that the following 

 propositions may be considered as reasonable : 



(1.) The extraordinary modifications and series of shells found at Steinheim are in one 

 way exceptional, and owe their existence to exceptional conditions. 



(2.) These conditions appear to be the isolation of the modified descendants of PL 

 levis, due to the absence of competing types, and the character of the environment. 



(3.) This environment was suitable for the propagation and joerpetuation of the 

 distinctive peculiarities of three series, and unfavorable in various degrees to those of 

 the sub-series of the First Series. 



(4.) That while the perpetuation and survival of the differential characteristics can be 

 thus accounted for, we must look to other causes for the production of the parallel forms 

 and the regularity of succession of these forms, as shown in the arrangement in the 

 different series, and in the development of the individual. 



(5.) That this cause lies in some law of growth and heredity which reacts against the 

 tendency of the physical environment to j^roduce variations and differences, and produces 

 parallelism in the development of different individuals of the same species, of difl"erent 

 species in the same series, and in the succession of forms in the diff'erent series, and 

 also limits the tendency to variation within definite boundaries in the species, espec- 

 ially in PL levis. 



(6.) That while the influence of the environment must be admitted as paramount in 

 exceptionable instances, it for the most part produces these exceptions in extreme cases of 

 parasitism, and the Steinheim shells are not parasites, and cannot be assumed to have been 



