2 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



be more or less divided up into smaller parts visible with the eye 

 or with the aid of a lens. The stomach, for instance, is found to 

 have a wall .consisting of certain layers or coats. So far we have 

 had to do with External Morphology and Anatomy. But, with the 

 assistance of the compound microscope, a further analysis is pos- 

 sible, and the special aim of Histology is to consider minute struc- 

 ture. Such an analysis demonstrates that the body of a Frog is 

 composed of an exceedingly large number of miscroscopic units, 

 cells, differing very much in size, shape, and form, like cells being 

 aggregated into masses known as tissues, such as muscular, 

 nervous, &c. There are, besides, other elements formed by or 

 from cells. It is usual to regard the cell as the morphological 

 unit, but the higher powers of the microscope prove that the cell 

 itself is most wonderfully complex. 



Morphology, however, in the modern sense, is not a mere de- 

 scriptive study, but endeavours to discover not only how things 

 are but also why they are. In the Frog, for example, there is a 

 slender tube, the pineal body, running from the brain to the roof 

 of the skull. In the tadpole it stretches right up to the skin, 

 but the formation of the skull-roof pinches off its end, which 

 remains in the adult frog as a " brow-spot." What, then, is the 

 meaning of the pineal body and brow-spot ? No light is thrown 

 upon this question by the study of the Frog only, but by employ- 

 ing the comparative method a definite answer can be given. It 

 is known that in certain lizards the pineal body has the structure 

 which is characteristic of eyes, and we, therefore, conclude that the 

 Frog's remote ancestors possessed an unpaired eye in the top of 

 the head, which has since disappeared, leaving an insignificant 

 rudiment. In numberless other instances Comparative Morphology 

 (including Comparative Anatomy, &c.) helps to clear up otherwise 

 unintelligible matters. 



(2) From the physiological standpoint, a Frog is a machine 

 capable of performing various kinds of work. Physiology is con- 

 cerned, in fact, with the uses of the different parts of the body. 

 It investigates, for example, the processes by which food is 

 digested, absorbed, and built up into living tissue; the circulation 

 of the blood ; and the functions of the brain. The problems of 

 physiology, like those of morphology, often require comparative 

 treatment, and they are commonly more difficult to solve. 



(3) Development (Embryology, Ontogeny) as applied to the 



