THE PROTOVERTEBRATE 23 



The sudden enrichment of the fossil record in the Cam- 

 brian is probably attributable not to any abrupt accelera- 

 tion of evolution but to the development of easily 

 fossilized, hard shells composed of chitin or calciimi 

 carbonate. The evolution of hard shells cannot be attrib- 

 uted to any sudden increase in either the calcium or the 

 carbonate content of the Cambrian seas, which proba- 

 bly had much the same composition as the sea today, 

 because both calciimi and carbonate had long been 

 abundant, and many of the Cambrian forms alternatively 

 made their shells of chitin, a nitrogenous material re- 

 sembling the stufF of fingernails and not requiring lime, 

 and still used by the modem insects and crustaceans 

 (the lobster, crab, etc.) to encase their bodies. It is more 

 likely that the accelerated evolution of the hard, external 

 skeleton reflects the spread of the carnivorous habit: 

 whereas free-floating, soft-bodied and defenseless animals 

 that fed on microscopic plants could survive without 

 armor in the sparsely populated sea, they began to eat 

 each other as their numbers and varieties increased, and 

 the advantage accrued to those that had developed 

 armor— which served to protect them against their fel- 

 lows. It is not surprising that simultaneously with the 

 appearance of protective armor, the invertebrates should 

 have evolved destructive mouth parts, prehensile limbs, 

 and, above all, increased strength and mobihty to pursue 

 their prey. The most advanced creatures in respect to 

 swimming, crawling and predaceous or scavenging habits 

 were the segmented eurypterids (see bottom of Figure 

 4), scorpionhke creatures allied to the present-day horse- 

 shoe crab, Limulus. These animals made up fully 60 

 per cent of the known Cambrian fauna, and, by the 

 multiplication of their jointed legs, demonstrate how the 

 ability to move about became, if not a sine qua non for 

 survival, certainly a distinct advantage. 



About one million species of living animals have been 

 described, of which those with backbones— the verte- 



