246 THE NATURE AND ORIGIN OF LIFE 



In any case, whatever is common to a n and a' n will be trans- 

 mitted to the egg resulting from the union between the two 

 parents and will inevitably comprise the characters acquired 

 by the adaptation of individuals of the same species to 

 identical conditions. Accordingly, with sufficient approxi- 

 mation we are able to neglect the complication of sexual 

 reproduction in recounting the Lamarckian history of the 

 progressive evolution of animals. 



The series of symbolic equations of specific evolution 

 has the same form as that of individual evolution. This is 

 expressed, in figured language, by the principle of Fritz 

 Miiller : 



" The embryology of an animal reproduces its genealogy." 



In other words, the series of individual characters appears 

 in the same order as the series of corresponding ancestral 

 characters. It is a fact of current observation that, when a 

 character has appeared in the parent at a certain age, the 

 same character appears in the child at a corresponding age, 

 that is, after other characters whose pre-existence in the 

 parent concurred in forming the given character. This 

 simple remark comprises the entire principle of Fritz Miiller. 



There may, however, be a simplification in the series of 

 embryonary forms. Instead of commencing at the stage 

 0,1 it may begin at a later stage a m and will then comprise 

 only the stages intermediary between a m and a n . This 

 takes place when, instead of leading a free life from the be- 

 ginning as its ancestors did, the embryo passes the first 

 stages of its existence inside an egg-shell or a matrix plenti- 

 fully supplied with food material. Then it issues forth at 

 the stage a m , and the stages previous to a m , in their condi- 

 tions of parasitic life, take on a new form Avhich is generally 

 much simpler as mechanism than the free forms a 1} a 2 , . . . 

 a m . It is then said that there has been embroygenic fore- 



