DIFFERENCES BETWEEN DIFFERENT ANIMALS. I 3 



as every plant 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 simply to the laws, 

 form, and arrangement of the structures of the organism ; in 

 short, to its external shape and internal structure. This con- 

 stitutes the science of morphology (^oop^ form, and Xoyo?, dis- 

 course). From the second, we have to study the vital actions 

 performed by living beings and \htfunctions discharged by the 

 different parts of the organism. This constitutes the science 

 of physiology. 



A third department of zoology is concerned with the rela- 

 tions of the organism to the external conditions under which 

 it is placed, constituting a division of the science to which the 

 term " distribution " is applied. 



Morphology, again, 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 before it assumes its mature or adult charac- 

 ters (embryology or development). The term " histology " is 

 further employed to designate that branch of morphology which 

 is specially occupied with the investigation of minute or micro- 

 scopical tissues. 



Physiology treats of all the functions exercised by living 

 bodies, or by the various definite parts or organs, of which 

 most animals are composed. All these functions come under 

 three heads : i. Functions of Nutrition, divisible into func- 

 tions of absorption and metamorphosis, comprising those func- 

 tions which are necessary for the growth and maintenance of 

 the organism. 2. Functions of Reproduction, whereby the per- 

 petuation of the species is secured. 3. Functions of Correlation, 

 comprising all those functions (such as sensation and voluntary 

 motion) by which the external world is brought into relation 

 with the organism, and the organism in turn reacts upon the 

 external world. 



Of these three, the functions of nutrition and reproduction 

 are often collectively called the functions of organic or vege- 

 tative life, as being common to animals and plants ; while the 

 functions of correlation are called the animal functions, as 

 being more especially characteristic of, though not peculiar to, 

 animals. 



6. DIFFERENCES BETWEEN DIFFERENT ANIMALS. 



All the innumerable differences which subsist between dif- 

 ferent animals may be classed under two heads, corresponding 

 to the two aspects of every living being, morphological and 



