HOMOLOGOUS ORGANS. 33 



Correlation of Organs. 

 Jt 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 circulatory 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 cor7-elated variation 

 in another, but sometimes we cannot trace the connection. 



Homologous Organs. 



Organs which arise from the same primitive layer of 

 tiie embryo (see Chapter IV.), 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. 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, re- 

 semble one another in being limbless, but this "convergence" 

 of form does not indicate any relationship between them. 



To describe such cases the term homoplastic is used. 



