36 Heredity. 



hitter! The developing impulse which in the one case 

 .is transfeiTcd from the ancestral species to the entire 

 group of species, and in the other case from the ances- 

 tral cell to the entire group of cells, assumes in both 

 cases the same form of a branching wave-motion. Any 

 one who accepts the fundamental law of development 

 will fiud it only natural that the microcosm of the onto- 

 genetic 'cell-tree' should be a diminution, aud to some 

 degree distorted reliection of the phylogenetic ' species- 

 tree. ' 



*'As we can only explain and render intelligible a 

 complicated and obscure phenomenon by*dividing it into 

 its separate elements, and by the exact analysis of these 

 parts, so it is necessary to penetrate to the ultimate 

 elementary facts of our mechanical theory of develop- 

 ment. 



'' Now the biogenetic process as a whole is the highly 

 compound resultant of the developmental history of all 

 species of organisms. These consist again of the life 

 histories of the individuals, just as the lattei' are again 

 made up of the histories of the constituent i)lastids. 



*' The development of each plastid, however, is in its 

 turn only the product of the active movements of its 

 constituent plastidules. Xow we have seen tliat the de- 

 r. yelopmental impulse of the branches and classes, the 

 orders and families, the genera and species, the individ- 

 uals and plastids, always and everywhere has for its fun- 

 dimcntal characteristic the branched wave-motion. Ac- 

 cordingly the molecular plastidule-motion, which lies at 

 the bottom of all the phenomena of life, can have no 

 other form. AYc must conclude that this ultimate cause 

 of all the phenomena of life, that the invisible activity 

 of the organic molecules is a branched wave-motion. 

 This true and ultimate causa efficiens of the biogenetic 



