110 EVOLUTION AND DOGMA. 



tebrates, which are more familiar to the general 

 reader, we observe that all the members of this ex- 

 tensive group are constructed on the same general 

 type. They belong, as it were, to the same style of 

 architecture, and we can trace the variations of 

 structure of similar parts with ease and precision. 

 They are all descendants of but one archetypal 

 form, of one primal vertebrate, from which all 

 others are derived by adaptive modification. This 

 is beautifully illustrated in the homologies of the 

 vertebrate skeleton. 



And here it is necessary to remark that analo- 

 gous organs are by no means homologous organs. 

 Analogous organs are those which are similar in 

 form and function, but of different origin. Homol- 

 ogous organs, on the contrary, are those which, 

 however different their form and functions, can be 

 shown to have community of origin. Thus, the 

 wings of birds and butterflies are analogous, but 

 not homologous. They have the same general 

 form and function, but they have not the same 

 origin ; that is, they have not been produced by 

 modification from the same organ or part. On the 

 other hand, the arms of men and apes, the fore-legs 

 and fore-paws of mammals and reptiles, the wings of 

 bats and birds, and the paddles of cetacea and the 

 breast-fins of fishes are homologous, because, how- 

 ever diverse their forms and functions, they can all 

 be demonstrated to have a common origin. They 

 have essentially the same structure and are com- 

 posed of the same pieces, although in view of their 

 diverse functions they are so modified that the 



