RETROGRESSIVE DEVELOPMENT. 28 5 
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 
phenomena of differentiation, which are of direct advantage 
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 degeneration. 
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 
individual organism. A differentiation of parts, which 
