30 GENESIS OF THE ARIETIDZ. 
crook in the living chambers or the rostrum. These cases also illustrate Dohrn’s 
theory’ 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? 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 enviroment may assuredly be assumed to have been favorable in the case 
of the parallel series of normal forms of the Ammonitinz 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 Ceratitinse, and 
these in turn, as well as the Lytoceratinze and Ammonititine 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 Arietids 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 Planorbide at Steinheim. 
When the environment, however, became unfavorable to growth, we find 
retrogression and retrogressiye equivalence. Lobites is a genus of small species; 
Choristoceras, Cochloceras, and Rhabdoceras are also smaller than most of the 
Ceratitinz. 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 nepionic and nealogic characteristics in adults. 
The form and sutures of straight shells in the Jura and Cretaceous, for example, 
1 Der Ursprung und der Princip des Functionswechsel, Leipzig, 1875. 
2 Wachsthum’s Beding. d. Lym. stagnalis, Verhandl. d. Wurzb. phys. med. Gesell. N. F., IV.; also 
Naturl. Existenzbedin, d. Thiere, Leipzig, 1880; and Animal Life, ete., Appleton’s International Scientific 
Series, 1881. 
3 See also Preface, pp. iv. and v. 
eee caaaranaen 
