EXTERNAL AND INTERNAL EQUILIBRIUM 403 



grows, till its equilibrium can no longer be maintained 

 as a single system, and it divides, breaking up into fresh 

 units like itself. When these units separate reproduction 

 takes place, when they remain together a multicellular 

 body arises. The different cells of this, because they are 

 differently placed in regard to each other and to the 

 environment, are differently affected by each other and 

 by the environment, and differentiation begins between 

 the different cells. A new equilibrium tends to be 

 established, and we have a characteristic form and 

 structure developed. This must be in general harmony 

 with (i.e. in equilibrium with) the external conditions, 

 or the organism will die and disappear. Only those 

 forms which are in harmony with their surroundings 

 will survive, and it is only those which we see around 

 us. But the possible kinds of harmony are very varied, 

 for in the first place the protoplasm of different kinds 

 of organism is different, owing to the perpetual changes 

 which go on in the protein complexes that form the 

 basis of protoplasm, the perpetual readjustments of 

 internal equilibrium ; and in the second place the 

 external conditions are different for different individuals. 

 The constant interactions of one and the other are as 

 constantly bringing about fresh adjustments, new 

 positions of the total equilibrium of the whole organism 

 — in other words, different species of organisms. 



These are the causes of the immense variety in the 

 forms and structures of the organisms that we see 

 around us, and of the fact that they are in general, 

 and very often in the minutest particulars, wonderfully 

 well adapted to their surroundings. Otherwise they 

 could not come into existence. But alongside of, 

 coexisting with, the features of structure and function 

 that are thus adapted, we see many others which are 



