20 OUTLINES OF EVOLUTIONARY BIOLOGY 



Like other organisms, the Amoeba sometimes undergoes a period 

 of rest, during which its various activities are more or less com- 

 pletely suspended. Under these circumstances the pseudopodia 

 are withdrawn, the body is rounded off and a protective envelope 

 or cyst is secreted by the protoplasm. This, however, is only a 

 temporary state, perhaps necessitated by unfavourable conditions 

 of the environment, and sooner or later the organism emerges 

 from its retirement and resumes its activity. 



If in the course of its wanderings the Amoeba meets with an 

 abundant supply of food and takes in more than is actually 

 required to make good the waste of protoplasm ; if, in other 

 words, anabolism preponderates over katabolism, the organism 

 may increase in size by growth, by the addition of new particles 

 of protoplasm in excess of those which are used up. These new 

 particles are deposited, not on the surface, but throughout the 

 whole mass of protoplasm, between those which are already 

 formed. Thus growth takes place, not by accretion, as in a 

 crystal or a snowball, but by intussusception, and we have here 

 a characteristic though by no means absolute distinction between 

 the growth of not-living and that of living matter. 



As in all organisms, however, there is a limit to the size which 

 the body may attain, and this limit varies with different species 

 of Amoeba. It depends primarily, no doubt, upon the necessary 

 relation between surface and volume. As we have seen, all 

 interchange between the organism and its environment has to be 

 maintained through the surface, and a given area of surface 

 cannot supply the wants of more than a certain volume of 

 protoplasm. As the animal grows the volume must necessarily 

 increase in a much higher ratio than the surface, and the pro- 

 portion between the two is rapidly altered. This is probably 

 not the whole explanation of the limitation of growth in an 

 Amceba, the problem being doubtless complicated by other factors, 

 but we may take it as quite certain that increase beyond a certain 

 size, if possible, would inevitably result in death. Such a 

 calamity is avoided by the simple expedient of dividing into two 

 parts whenever the limit of safety is reached. The nucleus 

 divides first and the two halves move away from one another, 

 then the protoplasm constricts into a bridge between the two 

 nuclei, the bridge narrows and finally ruptures, and instead of 

 one Amoeba there are now two, each exactly resembling the 

 parent (Fig. 2, E.). 



