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monious development is brought about when the rate of development 
of each part of the body is exactly dosed and the duration of 
development exactly limited. This rate and this duration are different 
for each part and so in every stage of development there will be 
a different correlation between the organs. As an instance I mention 
the heart. In the tirst phase of embryonal development this organ 
grows very rapidly, and is already completely formed when other 
organs are stil more or less far removed from their final stages. 
For each organ comes the moment when it has reached its definitive 
size-relation to the whole, after which it further increases in size 
in accordance with the general growth. If, for instance, the size- 
correlation of the heart or the kidneys, as it exists in the beginning 
of the second month of development in the human foetus, would be 
made permanent, then individuals would be born with monstrously 
developed heart or kidneys. But in the course of the further develop- 
ment heart and kidneys decrease again in relative size. 
Now this same point of view also applies to the brain. It also 
increases enormously in size in the first developmental phase, not 
only in man, but in all Primates. Of all Primates the foetus passes 
a developmental stage in which with regard to the whole body the 
brain has a monstrous degree of development. It afterwards decreases 
relatively in size until a definite phase of development, after which 
regularly to take part in the general growth. Now the difference 
between man and the apes is, that this happens in man in an 
earlier stage of development than in the other Primates, therefore 
the correlative relation in him was fixed in a younger phase when 
the brain was relatively larger. So here again the principle of retard- 
ation finds expression. The sooner a limit is put to the relative 
diminution in size, the larger the brain will become, the longer this 
relative diminution lasts during the development, the smaller the 
definitive weight of the brain becomes. 
1 have done my best to give my view in as succinet a form as 
possible; I hope that the intelligibility has not suffered too much 
by doing so. 
Under the influence of which organs stood this retardative process ? 
In order to answer this question let us examine what is the con- 
sequence if the correlation is not fixed at the point of time normal 
for man, but, as in the other primates at a more or less later 
moment in the development, when the brain in regard to the entire 
organism has become relatively smaller. 
Then also in man, therefore, a decreased mass-relation of the 
brain to the whole is brought about, the wellknown microcephaly 
