28 THE ELEMENTS OF STRUCTURE. 



whole, but you cannot perform this experiment with the frOg. 

 Now, the structural result of physiological division of labour 

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

 complex, more heterogeneous. 



Contrast a bird and a sponge this time, and another great 

 fact about the evolution of organs is illustrated. Every one 

 feels that the bird is more of a unity than a sponge; its parts 

 are more closely knit together and more adequately sub- 

 ordinated to the life of the whole. We call this kind of 

 progress, integration. Differentiation involves the acqui- 

 sition 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 suffer, others also are affected. This is especially 

 true of certain organs which have developed and evolved 

 together, and are knit by close physiological bonds. Thus 

 the blood-vascular and the respiratory systems, the muscular 

 and the skeletal systems, the brain and the sense-organs, are 

 very closely united, and we say that they are correlated. A 

 variation, for better or worse, in one system often brings 

 about a correlated variation in another, but sometimes we 

 cannot trace the connection. 



Homologous Organs. — Organs which arise from the same 

 primitive layer of the embryo (see next Chapter), have 

 something in common. But when a number of organs arise 

 in the same way, from the same embryonic material, and are 

 at first fashioned on the same plan, they have still more in 

 common. Nor will this fundamental sameness be affected 

 though the final shape and use of the various organs be 

 very different. We call organs which are thus structurally 

 and developmentally similar, homologous. It obviously 

 makes no difference whether they belong to the same or to 

 different animals ; the nineteen pairs of appendages on a 

 crayfish are all homologous ; the three pairs of "jaws" in an 

 insect are homologous with the insect's legs ; and it is also 

 true that the fore-leg of a frog, the wing of a bird, the flipper 

 of a whale, the arm of a man, are all homologous. But the 

 wing of a bird and the wing of an insect are not really similar; 

 though they are both organs of flight, they are only analogous. 



