8 COMPAEATIVE ANATOMY. 



Anatomy explains tlie phenomena of Ontogeny. Ontogeny 

 by itself does not rise above the level of a descriptive discipline, and 

 in proportion to the exactness of its investigation possesses a value 

 as so much objective material. At the same time Ontogeny 

 gains scientific importance by its connection with Comparative 

 Anatomy. Its facts, v?hich by themselves are incomprehensible, or 

 are only teleologically explicable in a metaphysical sense, because 

 restricted to the later events in the history of an organisation, 

 are, by Comparative Anatomy, put in connection with the known 

 phsenomena of other organisms, and are thereby rendered explicable 

 phylogenetically. The necessity of an exact knowledge of Compara- 

 tive Anatomy for Ontogeny is suflficiently obvious. Just as little 

 can the former separate itself fi'om the latter : since from Ontogeny 

 Comparative Anatomy gains an insight into the lower stages of 

 organisation. To the same extent, and in the same way as On- 

 togeny helps to form the basis of Phylogeuy, does it render 

 indispensable service to Comparative Anatomy. 



A " Comparative Embryology " has sometimes been put in 

 contrast with Comparative Anatomy; of course merely as a theo- 

 retical division of the scope of study. A "comparative" Ontogeny 

 of this kind must, just as much as every individual ontogeny, 

 have regard to the organisation of the fully-developed stage ; and, 

 in fact, without Comparative Anatomy it cannot lead to any 

 scientific results. 



§ 8. 



The relations of every organism to the outer world in which it 

 lives, from which it obtains material, and to which again it gives it 

 up, cause the outer world to have an influence on the organism. 

 This influence is practically seen in the changes of the organism, 

 which depend further on a Variability which is inherent in it. 



Variability comes under our observation as the capacity for 

 adaptation, and in effect operates as a modifying and even metamor- 

 phosing agent upon the inherited organisation. 



The organism is altered according to the conditions which in- 

 fluence it. The consequent Adaptations are to be regarded as 

 gradual, but steadily progressive, changes in the organisation, which 

 are striven after during the individual life of the organism, pre- 

 served by transmission in a series of generations, and further deve- 

 loped by means of natm'al selection. What has been gained by the 

 ancestor becomes the heritage of the descendant. Adaptation and 

 Transmission are thus alternately effective, the former representing 

 the modifying, the latter the conservative principle. The endless 

 variation of the phaenomena of organisation is, we see, consequently 



