306 SCIENTIFIC THOUGHT. 



lishing of differences, and in reality " the embryo never 

 passes through the form of any other animal, but only 

 through the condition of indifference between its own 

 form and others." And he sums up his reflections by 

 stating that the " development of an individual of a 

 certain animal form is determined by two conditions : 

 first, by a progressive development of the animal by 

 increasing histological and morphological differentiation ; 

 secondly, by the metamorphosis of a more general form 

 into a more special one." 1 



In order better to understand the difference which 



separates these various reflections, though breathing so 



17. much the air of the more modern theory of evolution, 



Von Baer's 



views in from later views, and to prepare for a real comprehension 



modern 



terms. o f ^he g rea t step taken by Darwin, it will be helpful to 

 resort to modern nomenclature. None of the terms of 

 that vocabulary which was invented by Darwin and his 

 followers to bring home to the popular mind the main 

 points of his revolutionary doctrine are to be found in 

 the earlier writings of von Baer. Nevertheless they 

 are useful in defining the views of the great naturalists 

 who preceded Darwin. Since we have become familiar 

 with the idea of the origin and the transmutation of the 

 different animal and vegetable species, we are accustomed 

 to apply the genetic view not only to the growth and 

 development of individual living things in nature, but to 

 everything else. When von Baer speaks of development, 

 when he tells us that " the history of development is the 

 true source of light for the investigation of organised 

 bodies," he means development in the narrower sense, 



1 Loc. cit., p. 231 ; transl., p. 220. 



