768 CONCLUSION xxxn. z- 



evidence that the genetic mechanism is more complicated in the more 

 elaborately organized later animals. In reptiles, birds, and mammals 

 each individual has a genetic constitution so specific that a piece of 

 tissue grafted from one individual to another of the same species (a 

 homograft) nearly always sets up an immunity reaction and is ulti- 

 mately destroyed. However, in urodeles such homografts are success- 

 ful, presumably because the genetic mechanism is less specific. 



3. The increasing complexity and variety of vertebrates 



The above comparison is not intended to be a complete analysis of 

 the organization of early and late chordates, but only an indication 

 that the difference between the two is in the greater number of diverse 

 parts and actions found in the later type. At every stage of the life- 

 cycle there are more alternative possible actions available and better 

 methods for selecting the appropriate one. In other words in higher 

 organisms more information passes through the system. It was sug- 

 gested in the first chapter that this greater complexity of the higher 

 animals enables their life to be carried on under conditions that would 

 have been impossible for the simpler ancestors. The survey of the 

 evolution of chordates has certainly shown that since the Cambrian 

 the chordate organization has invaded situations very different from the 

 sea surface in which it probably arose. It would not be profitable now 

 to recapitulate all the stages of this change — they have already been 

 described throughout the book. If we consider ecological niches in 

 detail the number of fresh situations invaded by vertebrate life is 

 almost as great as that of the species in the group. Among the earlier 

 changes were the transfer from the sea surface to other waters and to 

 the sea bottom. The entrance into fresh water must have called into 

 play many special mechanisms of adjustment. Development of jaws, 

 perhaps 350 million years ago, probably from the anterior branchial 

 arches, gave the possibility not only of eating new types of food 

 (including fellow fishes) but also of performing simple acts of 'handling' 

 of the environment. The heavy armour of the early types was given 

 up and the body form was then greatly improved from a hydrodynamic 

 point of view and with development of the air-bladder into a hydro- 

 static organ the fishes achieved their full mastery of the water. 



Meanwhile other fishes left the water, probably in the Devonian 

 period, rather less than 300 million years ago. At first they operated 

 with little modification of the method of life they had used in the water, 

 but they later developed all sorts of devices to meet the new conditions, 

 the earlier types dying out as the later developed. This process has 



