30 GENESIS OF THE ARIETID;E. 



crook in the living chambers or the rostrum. These cases also illustrate Dohrn's 

 theory 1 of change of function, and the effects produced upon organs thereby, 

 which has been of the greatest use in our researches. Semper' s researches and 

 experiments 2 explain changes in organisms in the same way, as probably caused 

 by changes in the surroundings which have led to the adoption of new habits, and 

 the consequent modification or suppression of already existing organs, and some- 

 times to the building up of entirely new organs or parts. It is interesting to 

 note, that our investigations, though necessarily confined to purely morpho- 

 logical phenomena, have led to theoretical results similar to the conclusions of 

 Dohrn, Semper, and others. 



We can account for the existence of the parallel series on the basis of the 

 following law of relation to the surroundings : 



The response or reaction of the forms of different series to the action of the ordinary 

 surroundings in the same habitat produced progressive morphological equivalence, when the 

 external influences were favorable to growth? 



The environment may assuredly be assumed to have been favorable in the case 

 of the parallel series of normal forms of the Ammonitinaa and other chambered 

 shells, whether occurring in India or Europe. The diversity of these causes 

 was very considerable, but it was not of such a nature as to imply a change 

 of habitat, or any fundamental change not favorable to the growth of the shell. 

 The average size of Goniatitinse is considerably below that of the Ceratitinaa, and 

 these in turn, as well as the Lytoceratinaa and Ammonititinaa of the Trias, are 

 smaller as a rule than the same suborders during the Jura and earlier Cretaceous. 

 The steady increase in size in all the progressive series of the Arietidae culminating 

 in the huge shells of Coroniceras shows this very plainly, as may be seen upon 

 consulting the Summary Plates, and the same is true of Planorbidae at Steinheim. 



When the environment, however, became unfavorable to growth, we find 

 retrogression and retrogressive equivalence. Lobites is a genus of small species ; 

 Choristoceras, Cochloceras, and Rhabdoceras are also smaller than most of the 

 Ceratitinaa. The deformed species of the bifurcatus series are smaller than the 

 normal bifurcatus. All of the scaphitoid shells are notably smaller than their 

 congeners, and though there are many large Crioceratites, Ancyloceratites, and 

 Baculites, there are, so far as we know, no exceptions to the rule in cases which 

 have been traced to close-coiled forms. Retrogression is also exhibited in the 

 decreasing size of the retrogressive forms of Agassiceras, Asteroceras, and 

 Oxynoticeras. 



In the pathological species with extremely retrogressive forms there is an 

 evident exhaustion of the normal powers of growth and development, and prema- 

 ture senility. This is shown in the uncoiling, destruction of the ornaments, and 

 often also by the retention of naapionic and nealogic characteristics in adults. 

 The form and sutures of straight shells in the Jura and Cretaceous, for example, 



1 Der Ursprung nnd der Princip des Functionswechsel, Leipzig, 1875. 



2 Wachsthum's Beding. d. Lym. stagnalis, Vevhandl. d. Wurzb. phys. med. Gesell. N. F., IV.; also 

 Naturl. Existenzbedin. d. Thiere, Leipzig, 1880; and Animal Life, etc., Appleton's International Scientific 

 Series, 1881. 



8 See also Preface, pp. iv, and v. 



