xxxii. is EVOLUTION OF ORGANIZATION 781 



organization appeared several times during the Mesozoic — the teeth, 

 jaw and skull bones, and limbs apparently evolving together. On the 

 other hand differing combinations are also undoubtedly possible; the 

 notoungulate *Thoatherium developed the limbs but not the teeth of 

 a horse; * Australopithecus had the legs but not the brain of a man. 



Unfortunately we have no very satisfactory techniques for describ- 

 ing the organization and its changes. All populations are homeostatic 

 self-reproducing systems. They are able to remain alive by selecting 

 from their repertoire of possible actions those that ensure survival. 

 This selection may be done either between the genes ('natural selection') 

 or between possible courses of morphogenesis ('functional adaptation') 

 or between possible actions of the neuromuscular system ('behaviour'). 

 We need means for measuring the information flow involved in these 

 selections and the amount that is stored in the memory system of the 

 species. 



Change in the genetic system (evolution) is one of the means adop- 

 ted to ensure homeostasis, and evolution is a feature that is essential 

 for prolonged maintenance of the organization. If storage powers are 

 effective and adequate such a system, as it receives information, must 

 presumably develop a widening repertoire of responses, both mor- 

 phogenetic and behavioural. The appearance of increasing complexity 

 as organisms become as we say 'more highly evolved' is a measure 

 of the extent to which they have found ways of encoding new in- 

 formation about the environment and so channeling it as to produce 

 responses that keep the species alive in the face of new risks. The higher 

 organisms are thus those that pass the greater amounts of information, 

 using more complex codes. Unfortunately we have little quantitative 

 information about genetic, morphogenetic, or neural codes to support 

 such an assertion. 



15. Summary of evidence about evolution of vertebrates 



The evidence about the history of vertebrates may be said, then, 

 to show us the following facts: (1) In all or nearly all populations 

 the organization of life-processes changes, though often only slowly. 

 (2) The later types of organization usually replace the earlier in any 

 one environment. (3) Evolutionary change is not always obviously 

 associated with environmental change, though it may be so. (4) When 

 different populations adopt the same habit of life they often develop 

 similar but not identical organizations. Probably no population is 

 stable either in numbers or in genetic or phenotypic characteristics. 

 The evidence suggests that in most species evolution is going on now 



