350 ALTERNATION OF GENERATIONS 



way the programme of development, the double number defining 

 the sporophyte generation. 



It must not be supposed, however, that the life history unfolds 

 entirely through the operation of forces within the organism, as 

 a watch runs on the strength of the wound-up mainspring. 

 While the organism is truly a machine, it is a machine which 

 is constantly influenced by forces from without which modify 

 its complex adjustments, and, above all, it is a self-perpetuating 

 machine which makes its own repairs. 



There are two prominent theories respecting the manner in 

 which an organism develops from an egg or other reproductive 

 cell. The first, called preformation, assumes that the characters 

 of the adult are preformed or represented in miniature by rudi- 

 ments or other structures in the protoplasm. Development is, 

 therefore, something like the unfolding of a bud, and the results 

 are determined by conditions within the organism. The second 

 theory, termed epigenesis, is not willing to grant such a compli- 

 cated architecture to protoplasm, but holds that development 

 is guided chiefly by conditions without the organism. It is 

 probable that the correct interpretation lies between the two 

 extreme views, that the cell does have a complicated structure 

 far beyond our present possibilities of knowledge, but that the 

 processes of development are largely guided and controlled by 

 outer influences. 



