36 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 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. 



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 this is not true of the frog. The struc- 

 tural result of this physiological division of labour is differ- 

 entiation. 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 subordinated 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 

 organism that its parts should be mutually dependent. The 

 organs are all partners in the business of life, and if one 

 member changes, others also are affected. This is especially 



