NATURE OF LIFE. 5 



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 the 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 alimentary 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. 

 Formerly, no doubt, the progress of science was retarded and 

 its growth checked by a too exclusive reference of natural 

 phenomena to a so-called vital force. Equally unquestionable 

 is the fact that the development of Biological science has pro- 

 gressed cotemporaneously with the successive victories gained 

 by the physicists over the vitalists. Still, no physicist has 

 hitherto succeeded in explaining any fundamental vital phe- 

 nomenon upon purely physical and chemical principles. The 

 simplest vital phenomenon has in it something over and above 

 the merely chemical and physical forces which we can demon- 

 strate in the laboratory. It is easy, for example, to say that 

 the action of the gastric juice is a chemical one, and doubtless 

 the discovery of this fact was a great step in physiological 

 science. Nevertheless, in spite of the most searching inves- 

 tigations, it is certain that digestion presents phenomena 

 which are as yet inexplicable upon any chemical theory. This 

 is exemplified in its most striking form, when we look at a 

 simple organism like the Amoeba. . This animalcule^ which is 



