ORGANS OF LOCOMOTION. 11 



the limits of a single order or even of a single family of the 

 Vertebrata, the case is wholly different. The wings of birds 

 have the same typical structure throughout the whole class, and 

 the same is true of the wings of the bats, and of the parachutes 

 of the flying reptiles. "Within the limits of these narrow groups 

 each organ acquires a quite different value. Their constant 

 occurrence and the similarity of their structure and development 

 make us suppose that they must have originated through the 

 modification of one or more simple organs in the parent form of 

 each family. It is the same with regard to the fins of fishes, 

 whales, and other Vertebrata. Here, as in the first example, 

 we see that the organ or member which, within the limits of a 

 small group, is a transmitted character, and helps in determin- 

 ing the affinity of the individual forms, appears as a character 

 of adaptation when we compare the great systematic groups 

 with one another. And choose what organ we will for this 

 comparison, the result will be the same ; parts which have little 

 or no value as characters of adaptation assume a conspicuous 

 diagnostic importance when we have to trace out the relations 

 of affinity under a wide systematic aggregate, because, within 

 the limits of the smaller groups, they may always be regarded 

 as hereditary. Thus, too, we arrive at the conclusion that the 

 distinction drawn, in the most recent zoology, between characters 

 of transmission and those of adaptation, has only a partial value : 

 for every organ which originated by adaptation, and in the first 

 instance was worthless for the determination of the relations of 

 affinity, may quite easily nay, must become transmissible by 

 inheritance if it is rendered permanent simultaneously with 

 improvement in other directions, in several varieties or species 

 all derived from the same parent form. But it may be trans- 

 formed into an hereditary character in a yet wider sense, particu- 

 larly when an organ which has originated by adaptation in one 

 single species, or even in one individual, is transmitted to a long 

 series of generations which branch off into different families, 

 though all descended from the one parent form. 



It is not difficult to show by an imaginary' instance how 

 such a change in the organ might be effected side by side with 

 permanence of the fundamental form. Suppose, for instance, 



