XI. 



ON THE EVOLUTION OF THE VERTEBRATA, PRO- 

 GRESSIVE AND RETROGRESSIVE. 



I. PRELIMINARY. 



IisT attempting to ascertain the course of evolution of the 

 Vertebrata, and to construct phylogenetic diagrams which shall 

 express this history, among the difficulties arising from deficient 

 information one is especially prominent. As is well known, 

 there are many types in all the orders of the Vertebrata which 

 present us with rudimentary organs, as rudimental digits, feet or 

 limbs, rudimental fins, teeth, and wings. There is scarcely an 

 organ or part which is not somewhere in a rudimental and more 

 or less useless condition. The difficulty which these cases present 

 is, simply, whether they be persistent primitive conditions, to be 

 regarded as ancestral types which have survived to the present 

 time, or whether, on the other hand, they be results of a pi'ocess 

 of degeneration, and therefore of comparatively modern origin. 

 The question, in brief, is, whether these creatures presenting 

 these features be primitive ancestors or degenerate descendants. 



In the first place, let us define the meaning of the word degen- 

 erate. This must be done first from a structural or anatomical 

 standpoint. Degeneracy may be defined as a loss of parts with- 

 out corresponding development of other f>ai'ts. All animals are 

 degenerate in some respect or another, as, for instance, the Mam- 

 malia in the small size of the pineal gland and of the coracoid 

 bone ; so that degeneracy, as a whole, can only be affirmed where 

 the sum of the subtractions is greater than the sum of the addi- 

 tions. Function of the parts must, however, be consulted in this 

 matter. We naturally regard sensibility as the highest of animal 

 functions, and mind as the highest form of sensibility. There- 

 fore development of organs of sensibility and sense and mind, con- 

 stitutes a better claim of progress than development of stomach 

 or of skin. Since motion is under the direction of sensibility, 



