RUDIMENTARY ORGANS. 79 



degree functional; the gill-slits in the embryo chick; 

 the aborted pistils found in staminate flowers ; all 

 these and numerous other instances will occur as 

 illustrations of rudimentary organs. All of these struc- 

 tures are, without the slightest question, homolo- 

 gous with similar structures well developed and 

 functional in other animals and plants. They are 

 of no use to their possessors ; they may be widely 

 distributed, and yet very variable, being found al- 

 most exactly alike in distinctly related species, while 

 of two closely related species, one may possess them 

 and the other have almost no trace of them. Fre- 

 quently they are of the greatest value in classifica- 

 tion. They must be comprehended in any explana- 

 tion of the organic world, and no theory of classifi- 

 cation can be for a moment entertained, which does 

 not have its explanation of these organs. 



The theory of types meets the question in a 

 manner which at first sight seems to be intelligible. 

 Animals are built according to certain plans, and it 

 is necessary, in order that the plan be complete, to 

 have all its parts represented. For symmetry of plan, 

 therefore, all of the usual parts must be present, but 

 since, in many cases owing to different habits, some 

 of them may be useless, they are quite naturally 

 permitted by the designer to be rudimentary. But 

 aside from the fact that the whole idea of types is 

 unsatisfactory, it is evident that in this case it is in- 

 consistent with itself. If all closely related animals 

 were thus supplied with the requisite rudimentary 

 organs, this explanation might do, but where one 

 animal possesses them and another almost like it in 



