8o Evolution and Adaptation 



because in its organization it contains the basis of a mammal, 

 just so much more must it be different from the hypothetical 

 one-celled amoeba, which has no other characteristics than 

 those that go to make up an amoeba. Expressed more gen- 

 erally, the developmental process in the many-celled organ- 

 isms begins, not where it began in primitive times, but as the 

 representation of the highest point which the organization 

 has at present reached. The development commences with 

 the egg, because it is the elemental and fundamental form in 

 which organic life is represented in connection with the 

 reproductive process, and also because it contains in itself the 

 properties of the species in its primordia. 



" The egg-cell of the present time, and its one-celled prede- 

 cessor in the phylogenetic history, the amoeba, are only 

 comparable in so far as they fall under the common definition 

 of the cell, but beyond this they are extraordinarily different 

 from each other." 



" The phyletic series must be divided into two different kinds 

 of processes : — First. The evolution of the species-cell, which 

 is a steady advance from a simple to a complex organization. 

 Second. The periodically repeated development of the many- 

 celled individual out of the single cell, representative of the 

 species (or the individual ontogeny), which in general follows 

 the same rules as the preceding ontogeny, but is each time 

 somewhat modified according to the amount to which the 

 species-cell has itself been changed in the phytogeny. 

 Similar restricting and explanatory additions to the biogenetic 

 law, like those stated here for the one-celled stage, must be 

 made in other directions. Undoubtedly there exists in a 

 certain sense a parallel between the phylogenetic, and the 

 ontogenetic, development. 



" On the basis of the general developmental hypothesis on 

 which we stand, all forms which in the chain of ancestors 

 were end-products of the individual development are now 

 passed through by their descendants as embryonic stages, and 



