Life and Its Conditions. 1 1 



or even impossible, to decide with positiveness whether we 

 are dealing with a plant or an animal. In the higher orders 

 of the two kingdoms there is no difficulty in reaching a 

 decision, the higher animals being readily separated from the 

 higher plants by the possession of a nervous system, of a 

 locomotive power which can be voluntarily exercised, and of 

 an internal cavity adapted for the reception and digestion of 

 solid food. No so-called nervous system or organs of sense 

 are possessed by the higher plants, although some of them 

 doubtlessly manifest conscious and intelligent action, nor 

 are they capable of voluntary changes of place, nor provided 

 with any definite internal cavity, their food being generally 

 fluid or gaseous. 



Descending the scale to the very bottom, we reach a class 

 of animals, the Protozoa, which cannot be separated in many 

 cases from the Protophyta by these distinctions, since many 

 of the former have no digestive cavity, nor the slightest trace 

 of a nervous system, while many of the latter possess the 

 power of active locomotion. As to external configuration, no 

 certain rules can be laid down for separating animals and plants, 

 many of the lower plants, either in their earlier stages, or in 

 their maturity, being exactly similar in form to some of the 

 lower animals. This is the case with some of the Algae, 

 which resemble very closely in form certain Infusorian ani- 

 malcules. Again, many undoubted animals, which are 

 rooted to solid objects in their adult state, are so plant-like 

 in appearance as to be popularly regarded as vegetables. 

 The Sea-firs, and the more highly organized Flustras or Sea- 

 mats, which are usually considered as sea-weeds by sea-side 

 visitors, are a few of many examples that might be taken 

 from the so-called Hydroid Zoophytes. No decided distinc- 

 tion between animals and plants can be drawn as to their 

 minute internal structure, both alike consisting of molecules, 

 of cells, or of fibres. Some decided, though not universal, 

 differences exist in chemical composition. Plants exhibit a 

 decided predominance of ternary compounds, or compounds 



