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222 THEORETICAL BIOLOGY 



the whole germ according to the rule of genesis. After the 

 critical point, growth is governed by the rule of function, 

 s^ Now the critical point is not passed by all organ-systems 

 at the same moment ; in especial, the cerebrum of mammals 

 shows an important retardation in its development. On the 

 other hand, Nissl was able to show that the roof of the skull 

 of a rabbit from which one cerebral hemisphere had been 

 removed at a very early stage, does not ossify, but becomes 

 a tough membrane. The skull-bone, the function of which 

 consists in protecting the brain, is put out of function by 

 the removal of its charge. Correspondingly, it is no longer 

 governed by the rule of function, but, like every functionless 

 tissue, is checked in its development or simply absorbed. 



The rule of genesis proceeds on its appointed course as 

 an independent natural factor as far as the critical point, 

 but no further. Its routes can be recognised by morphologists 

 through the signs of genesis, and these furnish points of sup- 

 port for determining the relationships of animals. The rule 

 of function, however, acts like a new broom, which sweeps 

 away all that is useless, and retains only what is necessary. 

 It is also an independent natural factor, which, according 

 to its own rule, absorbs what is useless, checks in their growth 

 the organs of the subsidiary functions, even suppressing the 

 development of their properties, but increases the organs of 

 the chief functions and develops them down to their last 

 details. 



An organ that functions has passed the critical point, 

 and it degenerates if, later on, it is put out of function. The 

 most exact information we have about the degenerative pro- 

 cesses concerns the nerves. Accordingly, it is very striking 

 that, judging from Nissl's discoveries, the central nervous 

 system of the higher animals forms an exception in this re- 

 spect. The half cerebrum of a young rabbit which has been 

 deprived of all its nervous connectjoji both with receptors 



