EETEOGPvESSIVE DEVELOPMENT. 285 



are no perfecting, but which are rather the contrary, that 

 is retrogressions or degenerations. It is easy to see that the 

 changes which every species of animal and plant experi- 

 ences cannot always be improvements. But rather many 

 i)henomena of differentiation, which are of direct advantao^e 

 to the organism itself, are yet, in a wider sense, detrimental, 

 inasmuch as they lessen its general capabilities. Frequently 

 a relapse to simpler conditions of life takes place, and by 

 adaptation to them a divergence in a retrograde direction. 

 If, for instance, organisms which have hitherto lived inde- 

 pendently accustom themselves to a parasitical life, they 

 thereby degenerate or retrograde. Such animals, which 

 hitherto had possessed a well-developed nervous system and 

 quick organs of sense, as well as the power of moving freely, 

 lose these when they accustom themselves to a parasitical 

 mode of life; they consequently retrograde more or less. 

 There the differentiation viewed by itself is a degeneration, 

 although it is advantageous to the parasitical organism. In 

 the struggle for life such an animal, which has accustomed 

 itself to live at the expense of others, by retaining its eyes 

 and apparatus of motion, which are of no more use to it, 

 would only expend so much material uselessly ; and when 

 it loses these organs, then a great quantity of nourishment 

 which was employed for the maintenance of these parts, 

 benefits other parts. In the struggle for life between the 

 different parasites, therefore, those which make least preten- 

 sions will have advantage over the others, and this favours 

 their de2:eneration. 



Just as this is found to be the case with the whole 

 organism, so it is also with the parts of the body of an 

 iidividual organism. A differentiation of parts, which 



