SOME CONSEQUENCES OF EVOLUTIONARY RELATIONSHIP 365 



3. Within each phylum the "higher" forms undergo transformations 

 that are similar to and follow the same sequence as those of the "lower" 

 forms. 



4. These "higher" forms attain their greater final complexity by (a) 

 the addition of changes at the end of the original sequence and (b) the 

 modification of earlier embryonic stages that they share with the "lower" 

 forms. 



5. The changes that every individual undergoes during its develop- 

 ment from zygote to adult (ontogeny) run approximately parallel to the 

 changes that occurred in the evolution of its ancestral stock (phytogeny). 1 

 This conclusion constitutes the principle of recapitulation. It holds true 

 only in a very general way, as we shall see. 



The meaning of recapitulation. To illustrate what is meant by 

 recapitulation, let us consider the embryology of the frog. Beginning with 

 the zygote, the frog embryo follows the beaten track of vertebrate devel- 

 opment which we have already described. If we compare its successive 

 stages with the adult forms of other organisms, the following series of 

 structural resemblances can be seen: 



Frog Other Organisms 



Zygote Single-celled Protista 



Blastula Spherical colonial Protista 



Gastrula Coelenterate, such as Hydra 



Very early tadpole Primitive chordate, such as Amphioxus 



Later legless tadpole Fish 



Late tadpole, with legs Salamander (primitive amphibian) 



Adult frog (unique) 



This does not mean that a frog is successively a protozoan, a coelen- 

 terate, a primitive chordate, a fish, a salamander, and finally a frog. It is 

 truly a frog from the beginning, yet it does show these successive resem- 

 blances to organisms more and more advanced in the evolutionary scale 

 —organisms which correspond structurally with ancestral stages in frog 

 evolution. The geological record makes it certain that frogs came from 

 salamanderlike amphibians, that amphibians evolved from fishes, and 

 that fishes developed from more primitive chordates. There is no such 

 direct evidence as to the earlier steps that led to the chordates, but the 

 embryonic changes strongly suggest what some of them were. 



Why should a frog (or any other organism) follow this long, complex, 

 predetermined path of development and in so doing take on the temporary 

 likeness of ancestral forms? And why is this likeness only partial? Perhaps 



^'Ontogeny" (Greek on, "that which is," and genesis, "origin") refers to the 

 origin of the individual. "Phylogeny" (Greek, phylon, "race," and genesis) refers to 

 racial origins. 



