252 TOWARDS THE HUMAN FORM 



Why did these splendid creatures disappear ? For long ages 

 they had ruled the sea. Their shells have been preserved in such 

 great numbers that all their variations can be followed, and 

 their genealogical tree be worked out so completely that the 

 history of this group may be considered as an irrefutable 

 demonstration of the modification of living forms. Yet puissant 

 and plastic as they were, they became extinct. Are we to 

 think that the Ammonites needed such specialized food that 

 at some given moment it was not forthcoming ? Had this been 

 so Palaeontology would have given us at least some indication 

 of the facts, and we have none. Is it conceivable that some new 

 carnivorous marine animal of greater activity multiplied and 

 caused such a carnage among the Ammonites that they were 

 annihilated ? We shall see later that this was not impossible. 

 Side by side with this solution, however, the manifest 

 simplification of form and the profound alterations in certain 

 of them suggest some modifications in the environment in 

 which these beautiful molluscs prospered and elaborated their 

 structure until it proved insufficiently plastic to survive the 

 new conditions. As they were sea-animals, this change can 

 only have consisted in a lowering of the temperature, for the 

 sudden extinction of an organic type in unvarying surroundings 

 is thoroughly improbable. 



The only Molluscs which disappeared together with the 

 Ammonites were the Belemnites, which were also dibranchiate 

 Cephalopods and very numerous. Their conical shell, short, 

 straight, and partitioned, terminated in front in a kind of a 

 large spoon-shaped shield, concave towards the base, and 

 behind in a calcareous point with the form and dimensions of 

 a cigar. Such perfect imprints have been found that all the 

 soft parts of the body are easily recognizable. The ink-sac 

 has been so well preserved that if its contents are crushed and 

 mixed with water it can still be utilized to make a wash- 

 drawing. These organisms were close kin of our Squid. 

 They must have been more active swimmers than the 

 Ammonites enclosed within their shells, and they could thus 

 easily prey on them. However, the most important part in the 

 wiping out of both was probably played by the Fishes. 



This was the period when bony Fishes had been evolved, 

 and were added to those whose skeleton was still cartilaginous. 

 These newcomers undoubtedly sprung from the ganoid Fishes 



