CHAPTER III. 



DIFFERENCES BETWEEN DIFFERENT ORGANISMS. 



Morphology and Physiology. — The next point which 

 demands notice relates to the nature of the differences by 

 which one organism may be separated from every other, and 

 the question is one of the highest importance. Every Hving 

 being, whether animal or vegetable, may be regarded from 

 two totally distinct, and, indeed, often apparently opposite, 

 points of view. From the first point of view we have to 

 look solely to the laws, form, and arrangement of the struc- 

 tures of the organism ; in short, to its external form and 

 internal structure, wholly irrespective of the manner in 

 which it discharges its vital work. This constitutes the 

 science of Morphology (Gr. morJ)he, form ; togos^ a discourse). 

 From the second point of view we have to study the vital 

 actions performed by living beings, and the functions dis- 

 charged by the different parts of the organism, separately 

 or collectively. This constitutes the science of Physiology. 

 Morphology not only treats of the structure of* living 

 beings in their fully-developed condition (Anatomy), but is 

 also concerned with the changes through which every living 

 being has to pass in reaching its mature or adult condition 

 (Embryology or Development). The term " Histology," 

 again, is further employed to designate that branch of Mor- 

 phology which is specially occupied with the investigation 



