THE ORIGIN OF MAN 61 



were firmly established on the land we see them 

 going back again to the water, not only inter- 

 mittently to the lakes and rivers but permanently 

 to the seas and oceans where there was a more 

 certain food supply. We therefore see to what 

 a wonderful extent organisms can adapt them- 

 selves, for limbs have been changed from walking 

 legs to swimming paddles, and the egg-laying 

 method of rearing the young has been altered to 

 that in which the young are born alive. As to 

 the marine invertebrates the seas swarmed with 

 ammonids in great variety. The bivalves and 

 siphonate were in the ascendency though still 

 common in some places were as a rule very rare. 

 We shall now make a study of the dinosaurs, the 

 rulers of the Mesozoic, before entering the Juras- 

 sic period. 



There were two main groups of dinosaurs, one 

 herbivorous and the other carnivorous. The car- 

 nivorous dinosaurs, to which the name Theropoda 

 has been given and which means beast-footed, 



