14 THE EVOLUTION OF MAN. 



etc. It is evident that these organs appear earlier in 

 relation to others than was originally the case in the 

 history of the tribe. The reverse is true of the retarded 

 completion of the intestinal canal, the body-cavity, and the 

 sexual organs. It is evident that in these cases there is an 

 ontogenetic postponement or retardation. 



It is only by critically appreciating these kenogenetic 

 incidents in relation to the palingenetic, and by constantly 

 allowinof for the chano^es in inherited evolution effected 

 by vitiated evolution, that it is possible to recognize the 

 fundamental significance of the first principle of Biogeny, 

 which in this way attains its true value as the most im- 

 portant explanatory principle of the history of evolution. 

 When it is thus critically appreciated, this first principle 

 also proves to be the " red thread " on which we can string 

 every one of the phenomena in this wonderful domain ; 

 this is the thread of Ariadne, with the aid of which alone 

 we are able to find an intelligible course through this com- 

 plicated labyrinth of forms. Even at an earlier period, when 

 the history of the evolution of the human and the animal 

 individual first became somewhat more accurately known — 

 which is hardly half a century ago 1 — people were greatly 

 surprised at the wonderful similarity existing in the onto- 

 genetic forms, or the stages of the individual evolution, of 

 very different animals. They noticed also the remarkable 

 resemblance between these and certain developed animal 

 forms of allied lower groups. Even the older natural philo- 

 sophers recognized the fact that in a certain way these 

 lower animals permanently represent in the system of the 

 animal kingdom forms which appear transiently in the 

 evolution of individuals of higher groups. But formerly 



