^66 tHE EVOLUTION OF MAN, 



germ-layers, in the fission of these, in the construction of the 

 most important fundamental organs from these germ-layers, 

 etc. The first rudiments of the principal parts of the body, 

 and, above all, of the oldest main organ, the intestinal canal, 

 are everywhere originally identical ; they always appear in 

 the same simplest form. But all the peculiarities by which 

 the various larger and smaller groups of the animal kingdom 

 are difijrentiated from one another only make their appear- 

 ance gradually, and secondarily, in the course of the evolu- 

 tion of the germ ; and those which distinguish the animals 

 most closely allied in the system of the animal kingdom 

 are the latest to appear. This latter phenomenon can be 

 formulated as a definite law, which may be regarded as, in 

 some sense, an addition or appendage to the fundamental 

 law of Biogeny. It is the law of the ontogenetic connection 

 between systematically allied animal forms. The meaning 

 of this is that the nearer two full-grown perfect animals are 

 to each other in point of general body-structure, and hence 

 the more closely they are allied in the system of the animal 

 kingdom, the longer do their embryonic forms remain the 

 same, and the longer are their embryos, and their young 

 forms in general, either altogether indistinguishable, or dis- 

 tinguishable only by subordinate characters. This law 

 holds good of all animals in which the original form of evo- 

 lution has been correctly inherited palingeneticall3% or by 

 *' inheiited evolution ". Where, on the other hand, this ori- 

 ginal form has been altered kenogenetically, or by " vitiated 

 evolution," the law is less true in proportion as a greater 

 number of new evolutionary conditions have been intro- 

 duced by adaptation (cf pp. 10-14).^^^ 



If we apply this law of the ontogenetic connection 



