10 MANUAL OF ZOOLOGY. 



causes death; but there are some notable exceptions to this 

 statement. Thus, life may remain in a dormant or " potential " 

 condition for an apparently indefinite time, as exhibited by the 

 great tenacity of life, even under unfavourable circumstances, 

 exhibited by the ova of some animals and the seeds of many 

 plants. A still more striking example of this is afforded by the 

 minute microscopic animals known as the Rotifers or Wheel- 

 animalcules. These little creatures are aquatic in their habits ; 

 and diminutive as they are, they are, nevertheless, comparatively 

 speaking, of a very high grade of organisation. They possess 

 a mouth, masticatory organs, a stomach and alimentary canal, 

 a distinct and well-developed nervous system, a differentiated 

 reproductive apparatus, and even organs of vision. Repeated 

 experiments, however, have shown the remarkable fact that, 

 with their aquatic habits and complex organisation, the Roti- 

 fers are capable of submitting to an apparently indefinite de- 

 privation of the necessary conditions of their existence, without 

 thereby losing their vitality. They may be dried and reduced 

 to all appearance to dust, and may be kept in this state for a 

 period of years ; nevertheless, the addition of a little water will 

 at any time restore them to their pristine vigour and activity. 

 It follows, therefore, that an organism may be deprived of all 

 power of manifesting any of the phenomena which constitute 

 what we call life, without losing its hold upon the vital forces 

 which belong to it. 



Again, the vital resistance of the lowest organisms to changes 

 of temperature, seems to be in some cases much wider than 

 that stated above as generally true. Thus, the microscopic 

 organisms known as " vibrios " are stated to survive exposure 

 to a temperature of 300 R, and to be wholly unaffected by 

 being frozen ; whilst Dallinger and Drysdale have shown that 

 the germs of monads will survive exposure to temperatures of 

 from 280 to 300 F. The presence of oxygen in a free state, 

 too, though essential to the higher foTns of life, does not 

 appear to be necessary in the case of some of the lower; since 

 vibrios and bacteria appear to carry on a vigorous life in an 

 atmosphere of carbonic acid gas. Lastly, there are certain 

 conditions, such as the presence of sun-light, which are essen- 

 tial for the maintenance of life as a whole, though by no means 

 necessarily demanded for the life of individuals. Thus, vege- 

 table life is as a whole dependent upon sun-light, and though 

 animals can subsist in darkness, animal life is in reality de- 

 pendent upon plant-life, so that the total absence of the sun 

 would extinguish all life whatever. 



