BIOLOGICAL INVESTIGATIONS. 143 



The nepionic or baby stage has been found to have characters 

 derived from the more remote ancestors of the group or general 

 phylum of the class. It may be characteristic of a large genetic 

 division or order, or sometimes, perhaps, what many systema- 

 tists would call a class or subclass. These nepionic characters 

 often refer us back to earlier geologic time, or even to forms of 

 still unknown faunas. This brief statement is of course very 

 defective, but it is true in a general way and is in accord with 

 the common belief in the comparative invariability of embryonic 

 characteristics which immediately precede the nepionic in the 

 ontogenv. Whether this comparative invariability of earlier 

 ages, which forms the basis of most embryonic conclusions, be 

 considered as due to the similar conditions that may be sup- 

 posed to guard the earliest stages of every embryo among 

 Metazoa, or whether it be due to heredity, which necessarily 

 recapitulates the processes of rejuvenation derived from the 

 remotest ancestors more faithfully than those later acquired, 

 cannot be discussed here. It is, however, quite plain that 

 embryonic characters, as a rule, possess persistence in direct 

 proportion to their age, the most ancient being usually the 

 most persistent. 



The few examples of later acquired characters that have been 

 traced back through earlier and earlier inheritance to the embry- 

 onic stages are positive so far as they go, and enable us to see 

 that the variations that do take place in these earlier stages are 

 not the result of a different law from that which governs the 

 inheritance of later acquired characters, but probably the same. 

 It is well known that in the same class, or even order, embryos 

 of the same genetic stock may pass from the detailed represen- 

 tation of many successive stages in the formation of the three 

 layers, and of the body cavity and other parts (ex. Amphioxus), 

 to a comparatively complex abbreviation of what are supposed 

 to be the same processes, since they have identical results in 

 the development of adults. But so many details have been 

 lost, the blastula and gastrula for example, and other charac- 

 ters added, the food yolk as an illustration, that it becomes dif- 

 ficult to see that they are really modifications of identical 

 modes of development. If one is prepared to try the law of 



