228 THE THEORY OF EVOLUTION 



example, as the result of parasitism the entire altera- 

 tion shows itself between the first abnormal germ 

 stage and the completed form. 



4. Summary of the collected results. 



(1) We have seen how manifold are the alterations 

 to-day which underlie animals and plants. Direct 

 influences (stimuli) by climate, constitution of soil, etc., 

 isolation and close interbreeding through long periods, 

 adaptation to quite special modes of life such as Parasites 

 or Symbionts all these effect changes in a progressive 

 or retrogressive sense, i.e. in the shape of new formations 

 and more marked differentiation or of regression of 

 existing features. 



(2) None of the transformations observed or easily 

 to be regarded as such ever go so far that the allocation 

 to a certain type can no longer be made with certainty, 

 whether in the completed adult condition or in the 

 embryogeny, which indeed likewise forms a constituent 

 of the typical distinguishing character. There arise, 

 it is true, new species, genera, and even families, but 

 no animals and plants with an entirely deviating plan 

 of construction and higher total organization. With 

 the Parasites there remains at least a portion of the 

 embryogeny unchanged. 



(3) The alterations which we can determine in fossil 

 organisms are of the same kind as the present ones 

 i.e. they also carry in themselves the distinguishing 

 characters either of a direct adaptation to the environ- 



