History of the Theory of Heredity. 35 



evolution of each person is represented by a wave-line, 

 the entire tree will have the form of a branched wave- 

 motion, a ramified undulation. . . . 



"A natural system of classification is nothing but a 

 genealogical tree of allied species of organisms, and each 

 branch and twig of the tree corresponds to a greater or 

 smaller group of descendants from a common ancestral 

 form. This community of descent unites all the forms 

 of a class, an order, and so on. Since each class is di- 

 vided into various orders, each order into several families, 

 each family again into various genera, each genus into 

 a number of species and varieties, there is a similar 

 branching in the wave-motion which is carried from the 

 common ancestral form to the entire group of its de- 

 scendants; and each undulating branch implants in the 

 same way its individual motion on its various descend- 

 ants. 



"Now the fundamental law of embryology teaches us 

 that this history of the philogenetic evolution of organ- 

 isms is mirrored in miniature in the ontogenetic devel- 

 opment of each individual. Here the single waves an- 

 swer to the life of the constituent plastids (cytodes and 

 cells). The cytula, or the first segmentation cell which 

 originates from the fertilized egg, and out. of which the 

 many-celled organism is developed, bears the same rela- 

 tion to the various cell-generations which originate from 

 it by division, and which give rise later by specializa- 

 tion of function to the various tissues, that the stem- 

 form of a class or order bears to the various families, 

 genera and species which diverge from it, and which 

 have been differently evolved through adaptation to di- 

 versified conditions of existence. 



"The ontogenetic ' cell-tree' of the former has exactly 

 the same form as the philogenetic ' species- tree ' of the 



