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 Aydra, 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 Aydra will reproduce the 
whole animal, but this is not true of the frog. The struc- 
tural result of this physiological division of labour is difer- 
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 zntegration. 
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 
