208 GENERAL PHYSIOLOGY 



go through the same development. Since this is true of every 

 generation of parents and children, it must be true also of all the 

 ancestors of the race, even the earliest, i.e., the children are the 

 historic product of the whole racial development and in their 

 developmental history must pass through the whole history of the 

 race. 



But this is true only on the condition that heredity is the sole 

 factor that determines form. In such a case every minute pecu- 

 liarity that was once present in the ancestral series of the organ- 

 ism would repeat itself with painful exactness in the development 

 of the latter. Since individual development demands a relatively 

 short time and racial development shows an inconceivable variety 

 of changes in form, the remarkable spectacle would be presented 

 of the ontogeny of a higher animal appearing like the picture 

 in a constantly turning kaleidoscope, which never remains the 

 same but presents to the eyes at every moment a different form. 

 It is well known that this is not the case, but that the racial 

 development is recapitulated only in bare outlines and under- 

 goes manifold changes ; these latter are the cenogenetic phe- 

 nomena, which are caused by the second factor that determines 

 form, namely, adaptation. It has been seen that the form of 

 every organism is determined in a certain degree by external 

 conditions. Any form that lived at a certain geological period 

 in the racial series of an animal is, therefore, determined among 

 other things by the conditions that prevailed upon the earth's 

 surface at that period. The conditions now are entirely dif- 

 ferent. But not only have the conditions upon the earth become 

 different, but the animal in its development is under wholly differ- 

 ent conditions from the completed animal, especially if the first 

 developmental stages are passed through within the mother's body. 

 Since, however, these external conditions must effect an adaptation 

 of the organism in question, it is explained why in the ontogenetic 

 recapitulation of the phylogenetic series there appears not only 

 a simplification but also an alteration of certain phenomena. 

 Simplification comes about because developmental stages which 

 at the time of their appearance represented special adaptations to 

 certain conditions become bred out as useless and disturbing 

 factors now when those conditions are wanting; alteration occurs 

 by the adaptation of certain developmental stages themselves to 

 the new conditions. It is clear that here also selection controls 

 the change of form, and that characteristics arising cenogenetically 

 become transmitted like original ones. 



Accordingly, with Haeckel (75), the fundamental law of 

 biogenesis may be formulated in brief as follows : " Germinal de- 

 velopment is an epitome of racial development ; the more complete, the 

 more the abridged development is maintained ly heredity ; the less 

 complete, the more a falsified development is introduced ~by adaptation. 



