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, omologous. 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 doth 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 
