HISTORY OF ZOOLOGY. 55 



incapacity, which has increased from generation to generation. 

 Now we must believe that what is true for disuse must express 

 itself in the reverse sense in the case of use. 



Nageli's Principle of Progression. In conclusion, there is still 

 to be considered the change of species from internal .causes, to 

 which von Baer gave the poorly adapted because easily misleading 

 term " Zielstrebigkeit " (the striving toward an ideal), and which 

 Nageli has termed the ' perfecting principle/ or the ' principle of 

 progression/ It cannot, indeed, be denied that each species is 

 compelled, by some peculiar internal cause, to develop into new 

 forms, independently of the environment, and up to a certain 

 degree, independently of the struggle for existence. In all animal 

 branches we see the progress from lower to higher going on, very 

 often in a quite similar way, in spite of the fact that the animals 

 live under very different conditions of development. We see how 

 the nervous system lying near the surface in the lower animals 

 becomes in the higher animals concealed in the depths of the body; 

 how the eye, at first a simple pigment-spot, becomes in worms, 

 arthropods, molluscs, and vertebrates, provided with accessory 

 apparatus, as lens, vitreous body, iris, choroid, etc. Here we see 

 an energy for perfection which, since it occurs everywhere, must 

 be independent of the individual conditions of life, and must have 

 its special explanation in the character of the living substance. 



It is by no means justifiable to call an assumption, as here 

 expressed, teleological, and to reject it as unscientific; rather the 

 organism seems to be just as mechanically conditioned as a billiard- 

 ball, whose course is determined not only by contact with the 

 cushions of the billiard- table, but also in a large measure by its 

 indwelling force, imparted to it by the stroke of the cue. An 

 organism, too, is a store of energy which must necessarily from 

 itself develop more, but it is of more extraordinary complexity, 

 and to an equal degree also is independent of the external world. 

 A complete independence is naturally never present, and Nageli 

 has not so maintained. Along with it rather goes always an 

 'action' of the external world, a modifying influence which is 

 carried on by the external conditions of existence, either directly 

 or by the mediation of use and disuse. 



This exposition of evolution has been given in a rather detailed 

 way, because in the history of zoology it is undoubtedly the most 

 important feature. No other theory in the course of the develop- 

 ment of zoological investigation has gained such a hold, none has 

 propounded so many new problems and opened so many new fields 



