ORGANS. 37 



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 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. 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 but 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 ; 

 they are analogous but not homologous. 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 burrow- 

 ing amphibian, a burrowing lizard, and a burrowing snake 

 resemble one another in being limbless, but this " conver- 

 gence," 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 



