642 



HISTORY OF SCIENCE. 



saying that " All those individual animals and plants which can be 

 proved to have descended from one another or from common ancestors, 

 belong to one species." Compare these definitions with that given of 

 the species, etc., of plants on page 644. 



The doctrine of Descent by Natural Selection has shed a light upon 

 the interpretation of many groups of facts in natural history hitherto 

 involved in the deepest obscurity. It has often been remarked that 

 the colours of many animals are the same as that of their surroundings, 

 and that the resemblances extend even to marks, spots, or textures. 

 Good instances may be seen in any aquarium, as in the backs of flat 

 fish, which so exactly reproduce the colours and general aspect of the 

 gravel on which they lie, that the spectator must look attentively to 

 distinguish the fish from the stones. These resemblances are in some 

 cases carried to such a degree that the spectator is tempted to believe 

 that some ingenious artist had made copies of the objects imitated. 

 Striking examples of this " mimicry," as it has been termed, are pre- 

 sented by the "leaf-insects," 

 of which there are many 

 species. The aspect of 

 these creatures in colour, 

 markings, size, and texture, 

 coincides with that of the 

 leaves and twigs and moss 

 among which they live. One 

 species is represented in 

 Fig. 319; but the reader 

 who has never inspected the 

 specimens to be seen in 

 FIG. 320. -WALKING-STICK INSECT. museums will not be able 



to appreciate the closeness 



of the imitation. Fig. 320 represents a creaure which is called " the 

 walking-stick insect " (Bacteria trophinus), and will serve to show how 

 nearly an animal can simulate a slender twig. Mr. Wallace says : 

 " Some of these insects are a foot long and as thick as one's finger, 

 and their whole colouring, forms, rugosity, and the arrangement of 

 the head, legs, and antennae, are such as to render them absolutely 

 identical in appearance with dry sticks. They hang loosely about 

 shrubs in the forests, and have the extraordinary habit of stretching 

 out their legs unsymmetrically, so as to render the deception more 

 complete." 



It is not alone through general philosophic theories that biological 

 science has made the extraordinary advances which mark its history 

 in the present century. The physiologist is now provided with instru- 

 ments of precision by which to investigate his phenomena, and in 

 physiological research experiment goes hand-in-hand with observa- 

 tion. Much of the great advance in this region of science has un- 



