348 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



The assumed importance of the blastula and gastrula thus became the 

 foundation for HaeckeFs biogenetic principle of recapitulation. Starting with 

 the postulation that the hypothetical blastaea and gastraea represented the 

 adult phylogenetic stages comparable to the embryonic blastula and gastrula, 

 respectively, Haeckel proceeded, step by step, to compress into the embryo- 

 logical stages of all higher forms the adult stages of the lower forms through 

 which the higher forms supposedly passed in reaching their present state 

 through evolutionary change. The two-chambered condition of the develop- 

 ing mammalian heart thus became a representation of the two-chambered, 

 adult heart of the fish, while the three-chambered condition recapitulated the 

 adult amphibian heart, etc. Again, the visceral arches of the embryonic pha- 

 ryngeal regions of the mammal represented the gill-slit condition of the fish. 

 Ontogeny thus recapitulates phylogeny, and phylogeny of a higher species is 

 the result of the modification of the adult stages of lower species in the phylo- 

 genetic scale. The various steps in the embryological development of any 

 particular species, according to this reasoning, were caused by the evolutionary 

 history of the species; the conditions present in the adult stage of an earlier 

 phylogenetic ancestor became at once the cause for its existence in the em- 

 bryological development of all higher forms. Embryology in this way became 

 chained to a repetition of phylogenetic links! 



Many have been the supporters of the biogenetic law, and for a long time 

 it was one of the most popular theories of biology. A surprising supporter of 

 the recapitulation doctrine was Thomas Henry Huxley (1825-95). To quote 

 from Oppenheimer ('40): "One wonders how the promulgator of such a 

 distorted doctrine of cause and effect could have been championed by the 

 same Huxley who wrote: 'Fact I know and Law I know; but what is this 

 Necessity save an empty Shadow of my own mind's throwing?'." 



The Haeckelian dogma that ontogeny recapitulates phylogeny fell into error 

 because it was formulated upon three false premises due to the fragmentary 

 knowledge of the period. These premises were: 



( 1 ) That in evolution or phylogeny, recently acquired, hereditary charac- 

 ters were added to the hereditary characters already present in the 

 species; 



(2) that the hereditary traits revealed themselves during embryonic devel- 

 opment in the same sequence in which they were acquired in phylogeny; 

 and 



(3) that Darwin's concept of heredity, namely, pangenesis, essentially was 

 correct. 



The theory of pangenesis assumed that the germ cells with their hereditary 

 factors were produced by the parental body or soma and that the contained 

 hereditary factors within the germ cells were produced by gemmules which 



