1 66 ESSENTIALS OF BIOLOGY 



of the thing. The best example is that of the development of an 

 animal body. Thus a chick grows from an egg, but the form of 

 the undeveloped egg containing a blastoderm is very different 

 from that of the 6-days' old embryo, which again is different from 

 the newly hatched foetus. Yet here there is something that 

 remains the same in all these complex changes, and this we call 

 the '' organization " of the animal. 



Growth with differentiation is the process that we usually 

 observe. 



59. ON GROWTH IN INANIMATE THINGS 



Inanimate things grow by accretion of materials. Some natural 

 process leads to the formation of a thing and then to the increase 

 of this thing by continued process. Thus a river delta forms 

 when a river current becomes great enough to carry sand and mud 

 in suspension. As the river enters the sea the velocity of its flow 

 decreases and the suspended materials are deposited on the bottom. 

 The conditions are such that these deposited materials are laid 

 down as a thick sheet, narrow where the river current begins to 

 slacken and wide towards the sea. The accretion of materials 

 continues and so the delta grows while still retaining its general 

 form of a great, flat, expanse of sand and mud and swamp, 

 roughly triangular in shape. The precise shape varies as the river 

 channels change and as the suspended materials may change, still 

 the delta remains the same thing, being the result of certain 

 natural processes which continue. Its changes of form are 

 '* accidental," in the sense that they are due to small, and generally 

 independent causes which are, so to speak, " embroideries " upon 

 the main essential process. 



A sand-bank forms in a somewhat similar manner and undergoes 

 growth, with accidental changes in form. A volcanic cone grows 

 with the deposition, on its sides, of materials ejected from the 

 crater. These materials assume a certain '' angle of repose " 

 which depends on their nature. The crater may collapse when 

 there are violent eruptions and heavy lava outflows and the form 

 of the cone may become irregular. Still it increases in size and 

 retains a certain individuality, so that we speak, for instance, of 

 Vesuvius as being the same volcanic mountain now that it was 

 when the cities of the Plain were destroyed. 



