66 I The Process of Evolution 



vides considerable evidence for the various postulated processes and 

 systems. It seems clear that some feedback mechanism of this sort 

 must operate at the level of transcription of the DNA code and pro- 

 tein synthesis, just as such mechanisms are believed to be respon- 

 sible for developmental homeostasis at later stages. 



MODIFICATION OF THE 

 DEVELOPMENTAL SYSTEM 



Of all the phenomena of morphogenesis, none has received more 

 attention from evolutionists than so-called recapitulation. It was 

 soon observed by embryologists that early developmental stages of 

 vertebrates resembled one another ( at least superficially ) to a much 

 greater degree than did the adults. This has been interpreted by 

 some workers to mean that, in the course of development, each 

 organism goes through a condensed version of its phylogenetic 

 history— that man, for instance, goes through a one-celled stage 

 (zygote), fish stage (when gill pouches appear), a mammal stage, 

 etc. This generalization was originally called the biogenetic law by 

 Haeckel and is often stated as "ontogeny recapitulates phylogeny." 

 This crude interpretation of embryological sequences will not stand 

 close examination, however. Its shortcomings have been almost uni- 

 versally pointed out by modern authors, but the idea still has a 

 prominent place in biological mythology. 



The resemblance of early vertebrate embryos is readily explained 

 without resort to mysterious forces compelling each individual to 

 reclimb its phylogenetic tree. It first should be emphasized that an 

 early mammalian embryo resembles a fish embryo, not an adult fish. 

 Virtually all organisms begin development as a single cell. The great 

 diversity of life forms is the result of different courses of develop- 

 ment determined in large part by the sets of genetic information that 

 cause alterations of the course of development. However, each 

 change does not mean transformation of the developmental system. 

 These tremendously complex integrated systems may be successfully 

 modified only through accumulation of minor changes, with con- 

 comitant readjustments of balanced interactions of the various fac- 

 tors. By careful examinations of living and fossil organisms, we can 

 infer these gradual changes of developmental pattern. A good ex- 

 ample is the slow change in the vertebrate jaw structure, with the 

 articular and quadrate, which were parts of the jaw in mammal-like 

 reptiles, having been utilized as the ossicles of the hearing apparatus 

 in mammals. All such changes have involved the modification of a 

 preexisting developmental sequence and were possible only when 



