36 THE POPULAR SCIENCE MONTHLY. 



down of protein compounds ; but, as the formation of new protein within 

 the body of the plant usually exceeds this consumjDtion, plants, as man- 

 ufacturers of protein, are broadly distinguished from animals. It is 

 now known, however, that many very highly-organized flowering plants 

 are carnivorous, and digest and make use of the protein of the animals 

 which they capture, and it is probable that the potato-fungus and many 

 other parasitic plants obtain all their protein ready made, like animals; 

 and, as it is imjDossible to show that none of the lower animals have 

 the power to make protein for themselves, this distinction cannot be 

 made the basis of an absolute line between the two groups. 



The difference in the process of respiration in animals and plants is 

 well known. Animals while in a state of vital activity absorb oxygen 

 from the air ; and this is given off from their bodies, usually united 

 with carbon, as carbonic acid. The green plants, on the contrary, 

 absorb carbonic acid, which is separated by the chlorophyll, under 

 the influence of sunlight, into carbon, which is appropriated by the 

 plant, and oxygen, which is given off and may be again taken up by an 

 animal. This difference is made use of in the arrangement of an aqua- 

 rium ; enough green plants being placed in the water to absorb the 

 excess of carbonic acid given off by the animals, and to supply the 

 oxygen for their respiration. The difference is not by any means abso- 

 lute, however, since the vital changes of the plant are dependent, like 

 those of the animal, upon oxidation, and result in the formation of car- 

 bonic acid. The colorless plants, like animals, absorb oxygen and give 

 off carbonic acid. This is also true of green plants which are not ex- 

 posed to light ; but in the latter plants this process is normally masked 

 and hidden by the opposite process already spoken of. 



It is plain, from what has been said, that the separation of organ- 

 isms into two great groups — animals and plants — is convenient and 

 natural, and that the distinctions between them are real but not abso- 

 lute; and it is possible to define, that is give, all the characteristics 

 which are distinctive of an animal, without implying or assuming that 

 all animals conform with the definition to the same degree, or that no 

 plant shares any of the characteristics. Since the lower representatives 

 of the two groups resemble each other more closely than the higher 

 forms, and since all positive characteristics gradually disappear as we 

 approach the point of union or origin, we must, in order to give our 

 definition any definiteness whatever, neglect the lowest and simplest 

 forms, and consider only the more specialized. 



As shown by the highest forms, an animal may be described mor- 

 phologically as an organism made up of cells, which are usually without 

 a cell-wall or membrane. In the adult, the individualit}'- of these cells 

 is usually lost, since they are united to form membranes, tissues, and 

 fibres. In nearly all animals the tissues thus built up from cells fall 

 into four groups — epithelium, connective tissue, muscular tissue, and 

 nervous tissue. 



