PROCESSES OF EVOLUTION 245 



seen in groups whose entire evolutionary history lies in the past and is 

 now available for study. The group for which this theory was first 

 advanced, and which is still the most famous example of it, is the 

 Ammonitoidea, which were cephalopods flourishing from the Silurian 

 to the Cretaceous. They are now only represented by the allied stock 

 of Nautiloids. 



Hyatt (i 867-1903) claimed that the earliest representatives of the 

 ammonites had straight conical shells, and that the early stages of their 

 evolution consisted in the gradual acquirement of an increased curva- 

 ture by which the shell became coiled into a tight spiral, with the outer 

 whorls eventually overlapping and partly enclosing the inner whorls. 

 At the same time there was an increasing elaboration of the "suture," 

 which is the Hne along which the outer surface of the shell is met by 

 the septa which divide the coiled up cone into chambers. After reaching 

 a maximum of coiling and sutural-elaboration, the ammonites at the 

 end of their evolutionary course retraced their steps and became 

 straight with more simplified sutures. Having thus attained a racial 

 second childhood, the so-called "senescent" stage, they finally died out. 



The basis of observation on which this somewhat anthropomorphic 

 account is erected has recently been severely criticized, particularly by 

 Spath.^ He points out that the actual evidence for the temporal sequence 

 of forms showing progressively increasing coifing is very inadequate . 

 Actually there appear to have been straight, sHghtly coiled and tightly 

 coiled forms coexisting from the earfiest times. The suture line how- 

 ever was simpler in the early members of the group and became 

 more complicated later. During the greater part of the time when the 

 group was in existence, far the commonest form had a more or less 

 tightly coiled shell and elaborate suture, but loosely coiled forms were 

 always present. Shortly before the extinction of the ammonites in the 

 Cretaceous, an enormous evolutionary radiation occurred, and very 

 many new types were produced. Among these were straight shells with 

 reduced sutures, but there were also species which cannot be fitted 

 into the simple scheme, for instance, spirally coiled types. There 

 is thus no adequate evidence of an orderly evolution from simple 

 through complex back to simple forms. In fact, Spath states that if we 

 examine in more detail the particular sub-groups whose evolution can 

 be followed with certainty through fairly short periods, the evidence 

 suggests an evolution from tightly coiled forms to straight ones rather 

 than the reverse. 



^ Spath 1933. 



