ONTOGENY 27 



pare the ontogenies of five related species, the adults of which 

 represent an evolutionary series; species E has evolved beyond 

 D, F beyond E, and so forth. The first stage (i.e., the 

 fertilized ovum or zygote) of D is not merely a zygote (A), 

 but it is the zygote of species D, and consequently indicated in 

 our diagram by A D ; in its development this passes through 

 the specific stages B D , and C D , to the adult D. Species E 

 is more highly evolved than species D, but it begins its existence 

 as a fertilized ovum which again is specific, this time A E . In 

 its development to the adult form this may pass through stages 

 B and C similar to those of species D, but merely similar, not 

 identical, else the result would be D and not E. Therefore, we 

 call these intermediate stages B E and C E . Further, species E 

 may pass through a stage in some particulars resembling D; 

 this, however, does not exactly resemble D and is therefore 

 designated D E . Similarly for species F, G, and H; each is 

 more highly evolved than the preceding. Each passes through 

 stages which resemble stages in the development of the less 

 highly evolved species, yet each stage is really specific. 



Conditions of life change for the embryo as well as for the 

 adult, and if these younger organisms are to remain in existence 

 they must evolve to meet the changed conditions. The process 

 of evolution concerns not merely the adult, but the organism 

 at every stage of its existence. Stages such as B G or C H may 

 finally become so highly modified that they are no longer recog- 

 nizable as related to B and C, and might as well be termed 

 X G and Y H . It is then said that these traits are " coenogenetic 

 modifications" in distinction from " palingenetic characteris- 

 tics," which are obvious similarities to previous racial condi- 

 tions. But recognition of the idea that the entire life history 

 is undergoing evolution, at every point, very largely minimizes 

 the value of this very common distinction between coenogenetic 

 and palingenetic traits in development, for in a very true sense 

 all the traits of the developing organisms are in varying degrees 

 both coenogenetic and palingenetic. 



Finally, we see that the problem why the egg develops into 

 a form resembling its progenitors, rather than organisms of 



