6 MANUAL OF ZOOLOGY. 



remain certain conditions equally indispensable to the external 

 manifestation of vital phenomena; though life itself, or the 

 power of exhibiting vital phenomena, may be preserved for a 

 longer or shorter period, even though these conditions be ab- 

 sent. These extrinsic conditions of vitality are, firstly, a cer- 

 tain temperature varying from near the freezing-point to 120° 

 or 130°; secondly, the presence of water, which enters largely 

 into the composition of all living tissues ; thirdly, the presence 

 of oxygen in a free state, — this, like water, appearing to be a 

 sine qua non of life, though certain fungi are stated to offer 

 an exception to this statement.* 



The non-fulfilment of any of these conditions for any length 

 of time, as a rule, causes death, or the cessation of vitality; but, 

 as before remarked, life may sometimes remain in a dormant 

 or " potential " condition for an apparently indefinite length of 

 time. An excellent illustration of this is afforded by vhe great 

 tenacity of life, even under unfavourable conditions, exhibited 

 by the ova of some animals and the seeds of many plants ; but 

 a more striking example is to be found in the Rotifera, or Wheel- 

 animalcules. These are minute, mostly microscopic creatures, 

 which inhabit almost all our ponds and streams. 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 alimentarj'- canal, a dis- 

 tinct and well-developed nervous system, a differentiated re- 

 productive 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, with- 

 out thereby losing their vitality. They may be dried and 

 reduced to dust, and may be kept in this state for a period of 

 many 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. 



If, in conclusion, it be asked whether the term " vital force" 

 is any longer permissible in the mouth of a scientific man, 

 the question must, I think, be answered in the affirmative. 



are absolutely essential to the maintenance of life, probably even in it3 

 humblest manifestations. 



* Recent experiments, as ye unconfirmed, would go to prove that these 

 conditions of vitality are not of such essential importance. 



