10 MANUAL OF ZOOLOGY. 



higher plants, but to the lower, and it is in the very lowest 

 members of the vegetable kingdom, or in the embryonic and 

 immature forms of plants little higher in the scale, that we 

 find such a decided animal gift as the power of independent 

 locomotion. It is also in the less highly organised and less 

 specialised forms of plants that we find the only departures 

 from the great laws of vegetable life, the deviation being in 

 the direction of the laws of animal life. 



5. MORPHOLOGY AND PHYSIOLOGY. 



The next point which demands notice relates to the nature 

 of the differences between one animal and another, and the 

 question is one of the highest importance. Every animal 

 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 (juopcfrj?, form, and Xoyor, dis- 

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

 performed by living beings and the functions 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 

 characters (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 microscopical 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 : 1. 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 volun- 

 tary motion) by which the external world is brought into 



