42 THE ELEMENTS OF STRUCTURE 



and others. In a bilaterally symmetrical animal, such as 

 a flat-worm, the head is the region of greatest physiological 

 activity, e.g. most intense metabolism and greatest sus- 

 ceptibility to external influences. Behind the head the 

 activity decreases towards the relatively inert middle part 

 of the body ; the tail is a secondary centre of activity, less 

 intense than the head. It is possible to demonstrate these 

 gradients, and the physiological dominance of one region 

 over another, in many ways and in all types of animals, as 

 well as in organs and sometimes in single cells. These 

 studies provide a new point of view for the better under- 

 standing of many evolutionary processes, especially for 

 the nervous system, and of many problems of behaviour, 

 regeneration, etc. 



11. Organs. — We give this name to any well-defined 

 part of an animal, such as heart or brain. The word sug- 

 gests a piece of mechanism ; but the animal is more than 

 a complex engine, and many organs have several different 

 activities to which their visible structure gives little clue. 



Dijferentiation and integration of organs. — When we 

 review the animal series, or study the development of an 

 individual, we see that organs appear gradually. The 

 gastrula cavity — the future stomach — is the first acquisition, 

 though some would make out that it was primitively a 

 brood-chamber. To begin with, it is a simple sac, but it 

 soon becomes complicated by digestive and other out- 

 growths. The progress of the individual, and of the race, 

 is from apparent simplicity to obvious complexity. We 

 also notice that before definite nervous organs appear 

 there is diffuse irritability, before definite muscular organs 

 appear there is diffuse contractility, and so on. In other 

 words, functions come before organs. The attainment of 

 organs implies specialisation of parts, or concentration of 

 functions in particular areas of the body. 



If we contrast a frog with Hydra, one of the great facts in 

 regard to the evolution of organs is illustrated. Among the 

 living units which make up a frog, there is much more 

 division of labour than there is among those of Hydra. An 

 excised representative sample of Hydra will reproduce the 

 whole animal, but this is not true of the frog. The 

 structural result of this physiological division of labour is 



