DIFFERENCES BETWEEN ANIMALS AND PLANTS. 41 



SECTION III. 



DIFFERENCES BETWEEN ANIMALS AND PLANTS. 



49. At first glance, nothing would seem more widely 

 different than animals and plants. What is there in com- 

 mon, for instance, between an oak or an elm, and the bird 

 which seeks shelter amid their foliage ? 



50. The differences are usually so obvious, that this 

 question would be superfluous if applied only to the higher 

 forms of the two kingdoms. But this contrast diminishes, 

 in proportion as their structure is simplified ; and as we 

 descend to the lower forms, the distinctions are so few 

 and so feebly characterized, that it becomes at length dif- 

 ficult to pronounce whether the object we have before us is 

 an animal or a plant. Thus, the sponges have so great a 

 resemblance to some of the polypi, that they have generally 

 been classed among animals, although in reality they be- 

 long to the vegetable kingdom. 



51. Animals and plants differ in the relative predomi- 

 nance of the elements, oxygen, carbon, hydrogen and nitro- 

 gen, of which they are composed. In vegetables, only a 

 small proportion of nitrogen is found ; while it enters largely 

 into the composition of the animal tissues. 



52. Another peculiarity of the Animal Kingdom is, the 

 presence of large, distinctly limited cavities, usually intended 

 for the lodgment of certain organs ; such is the skull and 

 the chest in the higher animals, the cavity of the gills in 

 fishes, and of the abdomen, or general cavity of the body, 

 which exists in all animals, without exception, for the pur- 

 pose of digestion, or the reception of the digestive organs. 



53. The well-defined and compact forms of the organs 



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