ANIMALS AND VEGETABLES. 655 



Physiology has for its object the scientific co-ordination 

 of the phenomena and laws of life ; yet, writes Mr. Lewes, 

 " the attempts to define what we are to understand by Life, 

 have hitherto proved almost if not quite valueless." In 

 our previous investigations, we must have seen the value 

 and advantage of " studying Life in its simpler forms, 

 if Life is to be understood in its more complex ; and 

 no sooner do we comprehend the fact that the lower 

 animals present to us the more important phenomena of 

 Life under simpler forms and conditions, than we at once 

 recognise the study as indispensable." 



It was Ehrenberg who first asserted that there was an 

 absolute boundary between animals and plants ; finding 

 even, as he fancied he did, in the smallest of the former, 

 the Infusoria, which had previously been regarded as 

 mere unorganised masses of mucus, the same systems of 

 organs as those by which the most highly-developed animal 

 is characterised, that is to say, distinct nutritive, motile, 

 vascular, sexual, and sensitive systems. Siebold called 

 the existence of these organs in question, regarding the 

 organisation of the Infusoria as a homogeneous paren- 

 chyma, in which he recognised only a nucleus, and in one 

 division a mouth and oesophagus. Nevertheless he asserted 

 that plants and animals were essentially distinct, and that 

 there was no transition from one to the other, the nature 

 of the plant being always immotile and rigid, whilst the 

 animal possessed the faculty of contracting and expanding 

 its body. This contractility is, in his opinion, alone to be 

 taken as the characteristic feature. It is not, however, the 

 animal organisation itself which is contractile, but only a 

 single tissue in it ; all the rest, skin, bones, and connective 

 tissue, are as rigid or passive as the vegetable membrane, 

 or, at most, only elastic ; in the higher animals the muscles 

 only are contractile, and in those of the lowest classes, viz. 

 the Infusoria, the entire body. 



Whence Ecker assumed the existence of a special con- 

 tractile substance, which sometimes occurs in a formed 

 state, as a contractile cell or as muscular substance, some- 

 times amorphous, as in the bodies of the Infusoria, Rhizo- 

 poda, and Hydrozoa. Kdlliker confirmed this view, and 

 carried it out, particularly in the case of the Infusoria, 



