38 ON MAGNITUDE [ch. 



the body is more than the svm. of the properties of the cells of 

 which it is composed: as Goethe said, "Das Lebendige ist zwar 

 in Elemente zerlegt, aber man kann es aus diesen nicht wieder 

 zusammenstellen und beleben." 



Among certain lower and microscopic organisms, such for 

 instance as the Rotifera, we are still more palpably struck by the 

 small number of cells which go to constitute a usually complex 

 organ, such as kidney, stomach, ovary, etc. We can sometimes 

 number them in a few units, in place of the thousands that make 

 up such an organ in larger, if not always higher, animals. These 

 facts constitute one among many arguments which combine to 

 teach us that, however important and advantageous the subdivision 

 of organisms into cells may be from the constructional, or from 

 the dynamical point of view, the phenomenon has less essential 

 importance in theoretical biology than was once, and is often still, 

 assigned to it. 



Again, just as Sachs shewed that there was a limit to the amount 

 of cytoplasm which could gather round a single nucleus, so Boveri 

 has demonstrated that the nucleus itself has definite limitations 

 of size, and that, in cell-division after fertihsation, each new 

 nucleus has the same size as its parent-nucleus*. 



In all these cases, then, there are reasons, partly no doubt 

 physiological, but in very large part purely physical, which set 

 hmits to the normal magnitude of the organism or of the cell. 

 But as we have already discussed the existence of absolute and 

 definite limitations, of a physical kind, to the possible increase in 

 magnitude of an organism, let us now enquire whether there be 

 not also a lower limit, below which the very existence of an 

 organism is impossible, or at least where, under changed conditions, 

 its very nature must be profoundly modified. 



Among the smallest of known organisms we have, for instance, 

 Micromonas mesnili, Bonel, a flagellate infusorian, which measures 

 about -M fi, or •00034 mm., by •00025 mm.; smaller even than 

 this we have a pathogenic micrococcus of the rabbit, M. pro- 

 grediens, Schrdter, the diameter of which is said to be only -00015 

 mm. or -IS/x, or TS x 10"^ cm., — about equal to the thickness of 



* Boveri, Zellen-studien, V. Ueber die Ahhdngirjkeit der Kerncjrosse und Zellen- 

 zahl der Seeigellarven von der Chromosomenzakl der Ausgangszellert . Jena, 1905. 



