32 THE ELEMENTS OF STRUCTURE. 



II. ORGANS. 



We give this name to any well-defined part of an animal, 

 such as heart or brain. The word suggests 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. 



Differentiation and Integration. 



When we review the animal series, or study the develop- 

 ment of an individual, we see that organs appear gradually. 

 / The gastrula cavity the future stomach is the first acquisi- 

 tion, but 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 outgrowths. 

 The progress of the individual, and of the race, is from 

 simplicity to complexity. When we think over the animal 

 [ series 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. f 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. 



Contrast a frog with Hydra, and one of the great facts 

 about 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, but you cannot perform this experiment with the 

 frog. Now, the structural result of this physiological division 

 of labour is differentiation. The animal, or part of it, becomes 

 more complex, more heterogeneous. 



Contrast a bird and a sponge, and another great fact 

 about the evolution of organs is illustrated. The bird is 

 more of a unity than a sponge ; its parts are more closely 

 knit together and more adequately subordinated to the life 

 of the whole. We call this kind of progress, integration. 

 Differentiation involves the acquisition of new parts and 

 powers, these are consolidated and harmonised as the 

 animal becomes more integrated. 



