34 THE ELEMENTS OF STRUCTURE. 



credit of beginning the profitable habit of moving head-foremost ; had 

 some one not taken this step, we should never have known our right 

 hand from our left. 



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 com- 

 plex engine, and many organs have several different activities 

 to which their visible structure gives little clue. 



Differentiation 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 we cannot do this with the frog. The 

 structural result of this physiological division of labour is 

 differentiation. The animal, or part of it, becomes more 

 complex, more heterogeneous. 



If we contrast a bird and a sponge, another great fact in 

 regard to 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 subordinate to the life 

 of the whole. This kind of progress is called integration. 

 Differentiation involves the acquisition of new parts and 

 powers, these are consolidated and harmonised as the 

 animal becomes more integrated. 



Correlation of organs. — It is of the very nature of an 



