62 Elementary Biology. 



accelerate this motion, while excess of heat paralyses and 

 kills the organism. After a time the flagellum is gradually 

 withdrawn, the motion becoming slower and slower until 

 the amoeboid stage is again reached, and it becomes again 

 necessary for the organism to take in a stock of food to 

 make good the waste and enable it to remain in existence. 



The importance of these four stages in the study of the 

 interrelationships of the lower organisms, whether plant or 

 animal, and indeed in a study of the physiology of the 

 higher forms as well, cannot well be exaggerated. It will 

 be necessary, however, to devote a special section to the 

 discussion of the subject. 



SECTION III. THE RELATION OF UNICELLULAR TO 



MULTICELLULAR ORGANISMS. 



On reviewing the multiplicity of forms which we meet 

 with among the lowest plants and the lowest animals, 

 together with those organisms which are doubtfully classified 

 under either category, we soon discover that each and all 

 live for a longer or shorter period of their existence in one 

 or other of the four stages exemplified by the life-history of 

 Protomyxa, while most, if not all, show some indication 

 of a tendency to pass through a corresponding series of 

 stages in their own life-histories. One, for example, lives 

 during the greater part of its existence in the form of an 

 encysted cell ; but at another time, possibly only for an 

 exceedingly short period, the protoplasm escapes from its 

 cyst and becomes a motile flagellate organism. In other 

 cases the plasmodium stage is the one emphasised. In 

 many cases one or more stages of the general type of life- 

 history (that of Protomyxa) are omitted, or slurred over 

 very rapidly. Manifestly, if inquiry be made into the reason 

 for this emphasising of some one stage, the explanation will 

 be found in that adaptation of the organism to its environ- 

 ment which has already been discussed at some length in 



