vi. 8 TENDENCIES IN EVOLUTION 185 



on account of the large size of the eyes, which almost meet above the 

 brain, so that the diencephalon is long and thin. 



These strange creatures appear in the Jurassic, apparently de- 

 scended from the somewhat similar bradyodonts (such as *Helodus), 

 which were common in the Carboniferous and Permian. They pre- 

 serve some primitive features (vertebrae, jaw support, open lateral 

 line canals, cleavage) but have developed many specializations in the 

 teeth, operculum, fins, and brain, probably in connexion with life on 

 the bottom of deep seas. 



8. Tendencies in elasmobranch evolution 



The elasmobranchs have been in existence ever since the Devonian, 

 and for much of this long period of nearly 400 million years we can fol- 

 low their changes with some accuracy. This type of fish was first 

 formed by loss of the heavy bony armour of the earliest gnatho- 

 stomes, associated with the adoption of a rapidly moving and car- 

 nivorous habit. The resulting shark-like form has remained with 

 relatively little change through the whole history of the group; clado- 

 selachians from the Devonian are remarkably like modern sharks, and 

 it would be difficult to assert that the latter show clear signs of being 

 in any way of a 'higher' type. Both are in fact suited to the same mode 

 of life. 



If our interpretation of the evidence is right, however, the modern 

 shark type has been evolved from the Devonian type through a hetero- 

 dont stage. During the late Permian and Trias there was little fish 

 food for the sharks and they appear to have taken to living on inverte- 

 brates. Eating this diet was presumably easier for animals possessing 

 the two types of teeth described on p. 180, and the animals also 

 became rather flattened with their life on the bottom. On the 

 reappearance of numerous fishes in the sea, in the Jurassic, some of 

 these heterodonts resumed the shark-like habit, lost the crushing 

 teeth, and developed into the varied fish-eating types alive today. 

 Others of the heterodonts, however, became still more specialized for 

 bottom life, as the modern skates and rays. 



It is difficult to see any persistent tendency in all this, except to eat 

 other animals of some sort. When fishes are available sharks will eat 

 them, and the bodily organization for doing so seems to have been 

 evolved at least twice. Similarly other members of the same stock ate 

 molluscs and Crustacea and became modified for this. The tendency 

 is for survival or continuance of the animals and this leads them 

 to adopt whatever habits are possible given their surroundings. In 



