86 EVOLUTION AND NATURAL THEOLOGY. 



power of the dragon-fly's eye, when used by 

 the insect itself; but it would be highly in- 

 teresting, if it were possible, to make similar 

 experiments on the eyes of nocturnal insects, 

 or on those of insects whose colour-perceptions 

 are different to our own. The eyes of the 

 Vertebrata could scarcely be made the objects 

 of such experiments ; but as the visual powers 

 of diurnal insects appear to correspond ap- 

 proximately with those of diurnal vertebrates, 

 it is reasonable to suppose that the eyes of 

 nocturnal animals have also corresponding 

 powers. 



Another class of homological resemblances is 

 that which exists between the sexes. It is well 

 known that the special organs of generation 

 exhibit a very close homological resemblance, 

 and that they resemble each other so much in 

 the foetus, that its sex cannot be determined 

 until a comparatively late period. In fact, it 

 is now ascertained that both sexes (or to speak 

 more correctly, the potentialities of both) co- 

 exist in the foetus.* Many circumstances favour 

 the belief that the sex of a germ is not in- 



* Darwin, "Descent of Man," vol. i. p. 207. 



