INTRODUCTION. 5 



The growth arises from no internal necessity, as in organic bodies. The 

 bulk is not increased by a process of assimilation which converts the 

 unlike into the like. Minerals do not feed ; they cohere. Nor have they 

 any power of development. They pass through no definite cycles of 

 change ; they have no stages of growth, no ages, no power of repro- 

 duction. 



The constant round of actions, therefore, in the organized structure 

 called life, in them is wanting. They occupy space, but have neither 

 birth nor death. 



Distinction between Plants and Animals. — Organized bodies are 

 divided into two classes, — animals and vegetables, — constituting two sep- 

 arate kingdoms, which, though capable of ready recognition when studied 

 in their higher members, seem almost to overlap in their lowest expres- 

 sion. Hence, while the differences between the higher animals and higher 

 plants are so striking as not to need mention, when we examine the lowest 

 forms of life the greatest difficulty will sometimes be met with in the 

 attempt to decide whether the organism is an animal or a vegetable. For 

 when the protozoa, or lowest animals, are compared with the protophyta, 

 or lowest plants, all the differences which are so striking between the 

 higher animals and- plants are completely wanting ; yet the protozoa are 

 as truly animal as are the vertebrata,' and the protophyta just as surely 

 plants. Consequently the definition of an animal or a plant, to be of any 

 scientific value, must include the lowest as well as the highest forms. 



We found, in our comparison of organic and inorganic matter, that 

 differences in form could be clearly made out. The external charac- 

 teristics of plants and animals are, however, inadequate to distinguish 

 them. Many animal forms, such as the hydrozoa, are essentially plant- 

 like in their external form, growing from fixed points and even repro- 

 ducing themselves by " budding," — a process almost universally holding 

 in the vegetable kingdom. So also the well-known coral polyps and the 

 sponge closely resemble plants in external configuration, and, though 

 undoubtedly animals, were long placed by naturalists in the vegetable 

 kingdom. 



Then, on the other hand, many plants, examined in respect to their 

 external form alone, would often be confounded with animals. Thus, 

 the germs of many algae, the ciliated zoospores, are scarcely to be dis- 

 tinguished from infusorial animalcules. 



It was at one time thought that the power of motion was a proof 

 of animality ; but many of the lowest plants, such as volvox and the 

 diatoms, possess the power of motion, of changing their location, the 

 instruments being the same as in many animals, viz., cilia. Nor is the 

 power of moving in response to an irritant peculiar to animal life : 

 witness the Mimosa pudica, the sensitive plant, which closes its leaflets 



