66 GENERAL PRIA T CIPLES OF ZOOLOGY. 



jective causes, to which C. E. von Baer gave the poorly 

 adapted because easily misleading term " Zielstrebigkeit " 

 (the striving toward an ideal), and which Nageli has termed 

 the il perfecting principle" or the ''principle of progres- 

 sion."' It cannot, indeed, be denied that each species is 

 compelled, by some peculiar internal cause, to develop 

 into new forms, independently of the external conditions 

 of existence, and up to a certain degree, independently of 

 the struggle for existence. Ln 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 pig- 

 ment-spot, becomes in worms, Arthropods, mollusks, and 

 vertebrates, provided with accessory apparatus, as lens, 

 vitreous body, iris, choroid, etc. Herein 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 con- 

 ditioned as a billiard-ball, whose course is determined not 

 only by friction with the banks 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 

 reservoir of force which must necessarily from itself de- 

 velop more, only that it is of more extraordinary complex- 

 ity, and in 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. 



