XXXII. 2 (767) 



2. Comparison of the life of early chordates with that of mammals 



Such an early chordate is immensely complicated when considered 

 as a chemical system, yet it lacks the specializations that later became 

 so characteristic of vertebrates. The difference appears very clearly if 

 we contrast the organization and life of some such simple, amphioxus- 

 like chordate with those of a mammal. In almost every part of the body 

 of the latter we find cell types and organs that are not yet differentiated 

 in the former. For illustration of this difference we can look at almost 

 any tissue, say the skin, the blood, the gut, or the brain. In a mammal 

 the skin contains far more types of cell than are present in amphioxus; 

 there are hairs and these are different in various parts of the body; 

 there are several types of gland and of receptor organ. The blood, 

 again, circulates with great rapidity and in two circuits ; it is delicately 

 adjusted in composition so as to allow rapid flow of materials to the 

 tissues ; it contains haemoglobin in special corpuscles. In addition there 

 are numerous types of cell able to be used for defence, and a system of 

 antibodies for the same purpose that is almost certainly also far beyond 

 anything found in the earlier creature. Digestion in a mammal involves 

 an elaborate arrangement of mouth, oesophagus, stomach, and intes- 

 tine, each with a controlled pH and special masses of cells aggregated 

 into groups, such as the salivary glands, pancreas, and liver, the latter 

 a most elaborate chemical workshop. Finally the nervous system 

 possesses an enormous number of cells and elaborate receptor organs. 

 It gives the power to react to many aspects of environmental change 

 that cannot be discriminated by the simpler organism. Nervous con- 

 duction is rapid, allowing these large creatures to be well coordinated. 

 The nervous system, working through the many contractile parts that 

 are provided by the muscular system, enables the performance of 

 numerous elaborate acts, helpful in obtaining food, escaping enemies, 

 and perhaps particularly in providing for the care of young, which is 

 another characteristic mammalian feature. The pattern of behaviour 

 does not always follow one single course, but is adaptable and suited to 

 the conditions that are likely in view of past experience to be en- 

 countered. There is an elaborate system of chemical signalling by 

 many endocrine glands. 



No doubt this greater complexity found in mammalian organs 

 reflects a similar complication of the metabolic processes throughout 

 the body, though as yet we have little information about this. More- 

 over, an organism with so many diverse parts presumably depends for 

 its propagation on a genetical system that is very elaborate. There is 



