12 INTRODUCTION 



of their anatomy, to which Cuvier gave an immense impetus; 

 and lastly, the investigation of minute structure or histology 

 (Gk. histos, a texture), which, though the compound microscope 

 was invented near the end of the sixteenth century, practically 

 began as a serious study with Malpighi and others nearly a 

 hundred years later, and has rapidly advanced up to the present 

 time, every successive improvement of the microscope leading to 

 the acquisition of more extended and more accurate knowledge. 

 The microscope, too, besides vastly increasing our knowledge 

 regarding the structure of well-known animals, has thrown open 

 to investigation a once unsuspected world of minute forms, just 

 as the telescope has extended our field of observation in the 

 other direction. 



Morphology, however, is not content with merely cataloguing 

 facts regarding the structure of different sorts of animals. It 

 compares and classifies these facts, and endeavours, as far as may 

 be, to explain them in the light of the evolution theory. Innumer- 

 able problems are met with, many being of the most fascinating 

 description, and the solution of these constantly engages the 

 attention of numerous expert workers. Some of these questions 

 are naturally very abstruse, but many of them, when properly 

 presented, are certain to excite the interest and arrest the atten- 

 tion of almost any intelligent person. As a good example of 

 such a problem and its solution we may take the nature and 

 origin of teeth. 



Teeth, as seen, for example, in a dog or cat, are hard bodies 

 having a certain complex structure, and developed by the lining 

 of the mouth-cavity. The question may be asked — " How have 

 teeth been evolved, and are any other parts of the animal body 

 comparable with them?" No answer can be given to this unless 

 the comparative method be adopted (a method we owe to the 

 illustrious German biologist, Johannes Muller, and which is of 

 supreme importance in zoological matters) and a general survey 

 taken of the backboned animals generally. Mammals, birds, 

 and amphibians do not afford a solution of the problem; but on 

 coming to fishes we find that many of them possess numerous 

 hard defensive bodies in the skin known as "placoid scales", which 

 vary greatly in shape in different cases. Now these scales 

 greatly resemble teeth in structure, and if in a dogfish we closely 

 examine the neighbourhood of the mouth, it will be found that 



