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forms are not confined to special localities, as in the Jura, but are 

 found in all faunas so far as known. 



The facts show that some general physical cause acted simulta- 

 neously, or nearly so, over the whole of the known area of the 

 world during the Cretaceous period, and produced precisely similar 

 effects upon the whole type as had here and there been noticeable 

 only within limited localities and upon single species or small num- 

 bers of species during the previous periods. This general cause, 

 whatever it may have been, affected the type so as to cause the suc- 

 cessive generations of the larger part of the shells to become dis- 

 torted, smaller and more cylindrical in their whorls, smoother and to 

 lose their impressed zones and their complicated foliated sutures. 

 In extreme cases they became again, with the exception of the 

 earliest stages which are usually broken off and lost, perfectly straight 

 cones, like the orthoceratitic radicals. So much alike are they, that 

 it is quite common for those who are not students of this group to 

 mistake the degraded Baculites for the radical Orthoceras. This 

 decrease in size, increasing smoothness, and uncoiling, is precisely 

 parallel with the similar transformations taking place during old age 

 in the normal involute shells of the Jura, which, when old enough, 

 also depart from the spiral, or tend to straighten out, and always 

 lose their ornaments, decrease in size, and so on. 



The universal action of the surroundings, as we now know them, 

 is certainly not exclusively favorable to the continuance of life, and 

 may be wholly more or less unfavorable. It certainly perpetually 

 excites the animal to new and more powerful exertions, and, like 

 perpetual friction, wears out its structures by the efforts which it 

 obliges it to make for the support of the structures in doing work. 

 At first this leads to development, the supply being greater than the 

 demand ; but sooner or later, and with unvarying certainty, the de- 

 mand exceeds the powers of supply, and old age sets in, either pre- 

 maturely, or at the termination of the usual developmental periods. 

 The remarkable and at present unique example of the Ammonoidea 

 places us in a position where we can see the same process taking 

 place in the whole of a large group, with attendant phenomena 

 similar in every respect to those which we have observed in indi- 

 vidual shells of the same order. 



In numbers of species and genera, and in the complication of the 

 internal structures and the production of the external ornaments on 



