32 THE POPULAR SCIENCE MONTHLY. 



gards the three kingdoms — the mineral, the vegetable, and the animal — 

 as three successive steps in an ascending series, each being supposed 

 to have all the properties and characteristics of the groups below it, 

 and something new and entirely different in addition. This conception 

 admits of such brief and definite statement, and when put into words 

 is so clear and simple, that its general acceptance is quite natural ; and 

 we need not be surprised that it only gives way very slowly in favor of 

 a view which does not admit of the same definite and simple formu- 

 lation. 



We will now examine the reasons which have led modern thinkers 

 to reject this view, and to hold that, real and actual as the differences 

 between animals and plants are, they are by no means absolute. 



The most conspicuous and superficial difference between animals 

 and plants is the one which we have already noticed. Animals have 

 the power of free locomotion and of independent action, which is deter- 

 mined within the animal; these powers are called into action by changes 

 in the external world, and imply the existence of sensation and con- 

 sciousness. In all the ordinary plants the power of locomotion is lack- 

 ing, and there are no voluntary actions like those which result from the 

 sensitiveness of the animal. This difference finds its expression in the 

 well-known dictum of Linn^us : " Plantae vivunt ; animalia vivunt et 

 sentiunt ; " and ilpon it is based the classification of the functions and 

 organs of the animal as those of vegetative and those of animal life. 

 Those functions which are carried on independently of the will, and 

 are not influenced directly by changes in the external world — digestion, 

 assimilation, secretion, circulation, and reproduction, for example — are 

 called the vegetative functions of the body ; while those of relation, 

 such as sensation and voluntary motion, are called, in contradistinction, 

 the functions of animal life. 



The difference which has led to the general acceptance and current 

 use of these and many similar expressions is real, as far as the higher 

 and more familiar animals and plants are concerned, but, with the growth 

 of our knowledge of the lower forms of life, the necessity for expansion 

 and modification of the definition of both groups becomes apparent. 

 Many of the lower animals, such as the hydroids and sponges, as well 

 as many highly-organized animals, like the tunicates and oysters, lack 

 the power of locomotion; and, on the other hand, many of the lower 

 plants are quite actively locomotive. Certain plants which are by no 

 means low are quite sensitive to external changes, and the actions by 

 which they respond to these changes in the Venus's-flytrap, for in- 

 stance, give quite as good proofs of the existence of volition as are 

 afforded by the actions of many of the lower animals. 



Another superficial and easily-recognized distinction between ani- 

 mals and plants is afforded b}'' the contrast in general form and struct- 

 ure ; this, like the first, is real, as long as our attention is restricted to 

 the higher forms of the two groups. In the animal we find a sharply 



