12 MANUAI, OF ZOOLOOy. 



sphere is therefore characterised hy the production of free 

 oxygen. Animals, on the other hand, absorb oxvgen and emit 

 carbonic acid, so that their reaction upon the atmosphere is 

 the reverse of that of plants, and is characterised by the pro- 

 duction of carbonic acid. 



Finally, it is worthy of notice that it is in their lower and 

 not in their higher developments that the two kingdoms of 

 organic nature approach one another. No difficulty is ex- 

 perienced in separating the higher animals from the higher 

 plants, and for these universal laws can be laid down to which 

 there is no exception. It might, not unnaturally, have been 

 thought that the lowest classes of animals would exhibit most 

 affinity to the highest plants, and that thus a gradual passage 

 between the two kingdoms would be established. This is not 

 the case, however. The lower animals are not allied to the 

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

 bers of the vegetable kingdom, or in the embryonic and imma- 

 ture 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 (/iogpij, form, and Xoyos, dis- 

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

 performed by living beings and ihz functions discharged by the 

 different parts of the organism. This constitutes the science 

 of physiology. 



A third department of zoology is concerned with the relations 

 of the organism to the external conditions under which it is 

 placed, constituting a division of the science to which th6 term 

 " distribution " is applied. 



Morpholog)', again, not only treats of the structure of living 

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



