A DEFINITION OF EVOLUTION 



salt water, or to terrestrial habitats. Each requires different excretory 

 physiology, and the last also requires a cleidoic egg (encased in a shell) 

 and other adaptations for water conservation. Thus mutations can effect 

 embryonic as well as adult stages, and these, too, are subject to natural 

 selection, so that embryonic adaptations become part of the normal pat- 

 tern of development. 



The beautifully simple embryology of the echinoderms played an im- 

 portant role in the establishment of the Biogenetic Law. Yet the recent 

 comprehensive study of echinoderm embryology by Fell reveals extensive 

 differences among various groups of echinoderms, and these differences 

 are referable to embryonic adaptations. Fell even casts doubt on the 

 echinoderm-chordate relationship, for the hemichordate larva does not fit 

 into the scheme of larval relationships which he has worked out. Many 

 biologists, however, feel that this is a result of extensive modifications of 

 larvae by natural selection since the separation of their ancestors. 



Another difficulty of the Biogenetic Law was Haeckel's emphasis upon 

 resemblances of embryos to adult ancestors. Subsequent study has shown 

 that the resemblance is primarily between embryos of related animals, 

 and only incidentally does this sometimes suggest adults. Thus ontogeny 

 does recapitulate phylogeny in a significant way, but it is ancestral embry- 

 onic, not adult, stages which are repeated, and even these may be dras- 

 tically modified by adaptive mutations which are favored by natural 

 selection. 



Von Baer's Principles. In formulating his Biogenetic Law, Haeckel 

 started from von Baer's principles of embryonic differentiation, and these 

 are perhaps much sounder, a better guide to the embryological evidence 

 for evolution. These principles are the following: (1) General characters 

 appear in development before special characters. (2) From the more 

 general, the less general, and finally the special characters appear. (3) An 

 animal during development departs progressively from the form of other 

 animals. (4) Young stages of an animal are like young, or embryonic, 

 stages of lower animals, but not like adults of those animals. The attack 

 upon the Biogenetic Law has never produced (evidence that the findings 

 of embryology do not su]i]")()rt the facts of evolution: it has merely shown 

 that llaeckel and his followers read into it more than the data will sup- 

 port. A return to the principles of von Baer makes possible a reasonable 

 evolutionary interpretation of the facts of comparative embryology with- 

 out straining the evidence. 



REFERENCES 



Davis, I). Dwicm-, 1949. "C^onipaiatiM' Analoiiu and the E\oluti()ii of the \\\lv 

 brate.s." In "Genetics, I'alconlologv, and Im oluiion," edited bv lepson, Mavr, aiu 

 Simpson. Princeton UniversitN' I^rc-ss, Princeton, N.|. A caiefnl anal\sis of the role 

 ot comparatisc anatomy in enrrent studies on e\c)hition. 



De Beer, G. H., 1958. "Embryos and Ancestors," 3rd llcL, Oxford Universit\' I'rcss 

 A critical review, on a \ erv liroad basis. 



Fell, H. Bahhadough, 194S. "luhinoderm Enibr\ ()l()u;\ and the Origin ol (lie Ghor 

 dates," BioU><iical Rcvicus of llic C(inib>icl<j^c Pliilosophiral Socicti/, 23, 81-107 



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