ORGANS 



35 



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 the respiratory systems, the muscular 

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

 very closely united, 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. On the other hand, 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; 

 this is expressed by saying that they are only analogous. 

 Yet two organs may be both homologous and analogous, 

 e.g. the wing of a bird and the wing of a bat, for both are 

 fore-limbs, and both are organs of flight. Sometimes two 

 organs or two organisms — deeply different in structure — 

 have a marked superficial resemblance, simply because both 

 have arisen in relation to similar conditions of life. Thus a 

 burrowing amphibian, a burrowing lizard, and a burrowing 

 snake resemble one another in being limbless, but this 

 "convergence," or " homoplasty," of form does not indicate 

 any relationship between them. 



Change of function. — Division of labour involves restric- 

 tion of functions in the several parts of an animal, and no 

 higher Metazoa could have arisen if all the cells had 



