HOMOLOGOUS ORGANS 



43 



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 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 

 true of certain organs which have developed and evolved 

 together, and are knit by close physiological bonds. Thus 

 the circulatory and respiratory systems, the muscular and 

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

 very closely linked, and they are said to be correlated. A 

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

 about a correlated variation in another, though we cannot 

 always trace the physiological connection. 



Homologous organs. — Organs which arise from the same 

 primitive layer of the embryo (see Chapter IV.) have some- 

 thing 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. Thus 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. The wing of a bird and the arm of 

 man exhibit the same chief bones, blood vessels, muscles, 

 and nerves, and they begin to develop in the same way ; 

 they are homologous hut not analogous. The wing of a bird 

 and the wing of an insect, which resemble one another in 

 being organs of flight, are not the least alike in structure ; 



