56 EVOLUTION OF LIVING ORGANISMS. 



represented by a separate substance, present from the be- 

 ginning in the germ. Such a supposition would soon 

 lead us into that tangle of difficulties which have proved 

 fatal to the corpuscular theories of heredity. It is the old 

 controversy between the supporters of " epigenesis " and 

 "evolution" in development (evolution being used here 

 in the restricted sense of an unfolding during ontogeny 

 of parts already present on a small scale in the germ). 

 While the latter held that the parts of the adult are de- 

 veloped by the mere unfolding of corresponding elements 

 preformed in the germ, the " epigenetists " held that 

 they are formed de novo in every embryo from an un- 

 differentiated substance. In modern language we should 

 say that the structure of the adult is gradually developed 

 by a series of epigenetic changes, each one of which is 

 strictly determined by the stage preceding it, and strictly 

 determines that which follows it. So every feature of 

 a landscape, every detail of a melting view, is doubtless 

 strictly predetermined by the nature of the geological 

 strata and the surrounding conditions, but cannot be said 

 to have been preformed in previous geological epochs. 

 If every separately variable character had to be repre- 

 sented by a separate unit the number of units in every 

 gamete would become fantastically large. Moreover, 

 characters are variable independently at different stages 

 of growth not the butterfly only, but the chrysalis, 

 the caterpillar, and the egg may vary ; must there be a 

 unit for every stage of every character? Again, it is 

 difficult to account for the marshalling and development 

 of all these units in proper order without calling in the 

 aid of some mysterious governing force. These are some 

 of the difficulties which have led to the downfall of the 

 corpuscular theories of heredity. 



Great as has been the advance in our knowledge of the 

 inheritance of characters through breeding experiments 

 conducted on mendelian lines, there is some danger of 

 the factorial theory becoming a mere formal explanation 



