126 KARL ERNST VON BAER 



ncrcr rescmb/cs tlic adult of anotJicr animal form, but only its 

 embryo" (p. 224). 



These laws relating to development within the limits of 

 type are destructive of even a limited application of the 

 theory of parallelism, for not even within the limits of the 

 type is there a real scale which the higher forms must mount ; 

 each embryo develops for itself, and diverges sooner or later 

 from the embryos of other species, the divergence coming 

 earlier the greater the difference between the adult forms. 

 It is only because the lower less-differentiated adult forms 

 happen to be little divergent from the generalised or 

 embryonic type, that they show a certain similarity with the 

 embryos of the higher more differentiated members of the 

 group. Such similarity, however, is due to no necessary law 

 governing the development of the higher animals ; it is, on 

 the contrary, merely a consequence of the organisation of 

 these lower animals (p. 224). 



Von Baer goes on to show what are the distinguishing 

 embryological characters of the types and classes, working out 

 a dichotomous schema of development, which each embryo 

 must follow, branching off early or late to its terminal point, 

 according to the lower or higher goal it has to reach. 



One important consequence for morphology results from 

 von Baer's laws of differentiation within the type. If the 

 embryo develops from the general to the special, then the 

 state in which each organ or organ-system first appears must 

 represent the general or typical state of that organ within 

 the group. Embryology will therefore be of great assistance 

 to comparative anatomy, whose chief aim it is to discover 

 the generalised type, the common plan of structure, upon 

 which the animals of each big group are built. And the 

 surest way to determine the true homologics of parts will be 

 to study their early development. " For since each organ 

 becomes what it is only through the manner of its develop- 

 ment, its true value can be recognised only from its method 

 of formation. At present, we form our judgments by an 

 undefined intuition, instead of regarding each organ merely 

 as an isolated product of its fundamental organ, and discerning 

 from this standpoint the correspondences and dissimilarities 

 in the different types" (p. 233). Parts, therefore, which 



