48 GENERAL PRINCIPLES OF ZOOLOGY 



many generations have lived in darkness, are blind, either having no eyes, 

 or only vestiges of them, incapable of function. This seems to justify the 

 view that this condition is attributable to lack of use, since it has brought 

 about a functional and anatomical 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, which Nageli 

 has termed the 'perfecting principle,' or the 'principle of progression.' 

 It cannot be denied that each species is compelled, by some peculiar 

 internal cause, to develop into new forms, up to a certain degree inde- 

 pendently of the environment and of the struggle for existence. In all 

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

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

 organization is- so different in the various phyla. We see how the nervous 

 system, lying near the surface in the lower animals, becomes in the higher 

 animals internal; 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, chorioid. 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 reaction of the living substance to light. 



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 deter- 

 mined 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 it must necessarily 

 have developed from itself, but it is of more extraordinary complexity, 

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

 complete independence naturally never occurs. Instead there is 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 outline of evolution has been given in a rather detailed way, 

 because in the history of zoology it is the most important feature. No 

 other theory has gained such a hold, none has propounded so many 

 new problems and opened so many new fields for research. There is no 

 other zoological theory which compares with it in value as a working 

 hypothesis. To the objection that the theory is insufficiently grounded, 



