246 THE HISTORY OF CREATION. 
The correlative adaptation between the reproductive organs 
and the other parts of the body deserves a very special con- 
sideration, because it is, above all others, likely to throw 
light upon the obscure and mysterious phenomena of in- 
direct or potential adaptation, which have already been 
considered. For just as every change of the sexual organs 
powerfully reacts upon the rest of the body, so on the other 
hand every important change in another part of the body 
must necessarily more or less react on the sexual organs. 
This reaction, however, will only become perceptible in the 
formation of the offspring which arise out of the changed 
generative parts. It is, in fact, precisely those remarkable 
and imperceptible changes of the genital system (in them- 
selves utterly insignificant changes)—changes of the eggs 
and the sperm—brought about. by such correlations, which 
have the greatest influence upon the formation of the off- 
spring, and all the phenomena of indirect or potential adapt- 
ation previously mentioned may in the end be traced to 
correlative adaptation. 
A further series of remarkable examples of correlative 
adaptation is furnished by the different animals and plants 
which become degenerated through parasitic life or para- 
sitism. No other change in the mode of hfe so much 
affects the shapes of organisms as the adoption of a 
parasitical life. Plants thereby lose their green leaves; as, 
for instance, our native parasitical plants, Orobanche, La- 
threa, Monotropa. Animals which originally have lived 
freely and independently, but afterwards adopt a parasitical 
mode of life on other animals or plants, in the first place 
cease to use their organs of motion and their organs of 
sense. The loss of this activity is succeeded by the loss of 
