XXXII 



CONCLUSION. EVOLUTIONARY CHANGES 

 OF THE LIFE OF VERTEBRATES 



1 . The life of the earliest chordates 



We set out to try to define the features that are characteristic of 

 vertebrate life, hoping then to show how these features have changed 

 during evolutionary history. We may now summarize the evidence 

 collected and see how far it is possible to make general statements 

 about vertebrate life and the factors that change it. 



The vertebrate type of organization has proved capable of support- 

 ing life under a wide variety of circumstances; most of its modern 

 forms operate under conditions very different from those in which 

 the type first appeared. According to the most probable theory (p. 47) 

 chordate life began at the sea surface, as the ciliated larvae of some 

 creatures rather like sessile echinoderms. The first fish-like animals, 

 with the characteristic chordate organization, appeared when such 

 larvae acquired powers of rhythmic metachronal muscular movement, 

 in order to allow support of a large weight. We can still see approxi- 

 mately this stage today in amphioxus. It is not possible to summarize 

 the nature of this organization in any brief general statement. The 

 science of morphology is still young and ill equipped with general 

 principles; it does not allow us to define the varieties of living organiza- 

 tion with precision and completeness. At present we cannot give a 

 full description such as we might wish for, specifying the composition 

 and activities of an organism or the inherited code of instructions 

 under which it operates. We can only describe some of the methods 

 by which the system maintains itself, for example its means of nutri- 

 tion, respiration, and reproduction. 



The earliest chordates showed a rather low level of metazoan organi- 

 zation, with a relatively small number of distinct cell types and few 

 special organs. Nitrogen and other raw materials were obtained in the 

 form of minute plants, collected by ciliary action of the pharynx and gill- 

 slits. The food was broken down by a system of enzymes working in 

 alkaline solution, and absorbed through the walls of a simple intestine. 

 There was probably no specialization of cells of the walls of the gut to 

 produce enzymes or to perform particular operations of conversion or 

 storage; at least no special liver, pancreas, or other organs were present 

 for these purposes. There were no special respiratory surfaces and the 



