50 INTRODUCTION TO EVOLUTION 



In the first place, the number of ways in which an organism consisting of 

 multitudes of cells arranged in layers can arise from a single cell must 

 be limited. In part, then, the uniformity is imposed by those mechanical 

 and physiological necessities mentioned previously (p. 47). Such neces- 

 sities would operate to produce similarities in the broad outlines of devel- 

 opment. Similarities in details of development, on the other hand, are 

 more likely to have resulted from a second factor: inheritance from com- 

 mon ancestry. 



Theory of Recapitulation or Paleogenesis 



In its modern form this is the theory that the embryos of animals repeat 

 some of the developmental stages passed through by embryos of their 

 ancestors. 



The recapitulation theory has had a stormy history. The fundamental 

 idea stems from von Baer, whom we have already quoted. His theory 

 had substantially the form we have stated: "The young stages in the de- 

 velopment of an animal are not like the adult stages of other animals 

 lower down on the scale, but are like the young stages of those animals" 

 (de Beer's translation, 1958). Subsequently Ernst Haeckel formulated 

 the theory to mean that the embryos of higher animals repeat the adult 

 stages of their ancestors. This form of the theory was called the Biogenetic 

 Law, and was summarized by the statement: "Ontogeny recapitulates 

 phylogeny." Ontogeny is the life history of the individual, starting with the 

 ovum; phylogeny, as the term was used by Haeckel, is the series of adult 

 ancestors of the individual in question. Haeckel maintained that in some 

 way the adult condition of an ancestor is pushed back into embryonic 

 development so that embryos of descendants pass through that ancestral 

 adult stage. We shall see presently, for example, that in one stage the hu- 

 man embryo strongly resembles a fish embryo. Haeckel would not have 

 been satisfied with such a statement; he would have insisted that the hu- 

 man embryo at that stage resembles an adult fish. The recapitulation 

 theory was a great stimulus to research in embryology, but as investiga- 

 tion led to more complete knowledge of the subject it became evident that 

 Haeckel was wrong and that von Baer had been right. Embryos of higher 

 animals repeat embryonic stages of their ancestors, not adult stages. The 

 pros and cons of this intellectual conflict are ably set forth in de Beer's 

 Embryos and Ancestors (1958). 



Insofar as the recapitulation theory forms valid interpretation of facts, 

 we may look to embryonic development for clues as to the course taken by 



