4 INTRODUCTION. 



determine the animal or vegetable nature of any given or- 

 ganism ; and such, indeed, was the almost universal belief 

 of older observers. In point of fact, however, no hard-and-fast 

 line can be drawn, in the present state of our knowledge, be- 

 tween the animal and vegetable kingdoms, and it is often a 

 matter of extreme difficulty, or even wholly impossible, to 

 decide positively whether we are dealing with an animal or a 

 plant. So deeply has this difficulty been felt of late, that a 

 most able zoologist — Dr. Ernst Haeckel — has proposed to 

 form an intermediate kingdom, which he calls the Eegnum 

 Protisticum, and in which he proposes to place all organisms 

 of a doubtful character. Even such a cautious observer as 

 Professor Rolleston, while questioning the propriety of this 

 step, is forced to come to the conclusion that " there are or- 

 ganisms which at one period of their life exhibit an aggregate 

 of phenomena such as to justify us in speaking of them as ani- 

 mals, while at another they appear to be as distinctly vege- 

 table." In the case of the higher members of the two king- 

 doms there is no difficulty in arriving at a decision. The 

 higher animals are readily separated from the higher plants 

 by the possession of a distinct nervous system, of locomotive 

 power which can be voluntarily exercised, and of an internal 

 cavity fitted for the reception and digestion of solid food. The 

 higher plants, on the other hand, possess no nervous system or 

 organs of sense, are incapable of voluntary changes of place, 

 and are not provided with any definite internal cavity, their 

 food being wholly fluid or gaseous. 



The lower animals {Protozoa) cannot, however, be separated in many 

 cases from the lower plants (Proiophyta) by these distinctions, since many 

 of the former hare no digestive cavity, and are destitute of a nervous system, 

 and many of the latter possess the power of active locomotion. In determin- 

 ing, therefore, the nature of these ambiguous organisms, the following are 

 the chief points to be attended to : 



Firsdy, as to mere form or external configuration, no certain rules can 

 be laid down for separating animals and plants. Many of the lower plants, 

 either in their earher stages of existence or when grown up, are exactly 

 " similar in form to some of the lower animals. This is the case, for ex- 

 ample, in some of the Algce, which closely resemble some of the Infusoriau 

 animalcules. Many undoubted animals, again, are rooted to solid objects 

 in their adult state, and are so plant-like in appearance as to be always 

 popularly regarded as vegetables. This is the case with many of the 

 so-called hydroid zoophytes, such as the sea-firs, and also with the much 

 more highly organized sea-mats {Flustra), all of which are usually regarded 

 as sea-weeds by seaside visitors. This is also, but less strikingly, the case 

 with the corals and sea-anemones, of which the latter are often spoken of as 

 " sea-flowers." 



Secondly, no decided distinction can be drawn between animals and 



