730 



GENETICS AND EVOLUTION 



Fish Salamander Turtle Chicken. 



Pi^ 



Cow Rabbit Man 



Figure 35.5. Comparison of early and later stages in the development of verte- 

 brate embryos. Note the similarity of the earliest stages of each. 



in the course of development, repeat the evolutionary history of their 

 ancestors in some abbreviated form. This idea, succinctly stated as 

 "Ontogeny recapitulates phylogeny," stimulated research in embryology 

 and focused attention on the general resemblance between embryonic 

 development and the evolutionary process, but it now seems clear that 

 the embryos of the higher animals resemble the embryos of lower forms, 

 not the adults, as Haeckel had believed. 1 he early stages of all vertebrate 

 embryos, for example, are remarkably similar, and it is not easy to dif- 

 ferentiate a human embryo from the embryo of a fish, frog, chick or pig 

 (Fig. 35.5). In recapitulating its evolutionary history in a few days, weeks 

 or months the embryo must eliminate some steps, and alter and distort 

 others. In addition, some new characters have evolved which are adaptive 

 and enable the embryo to survive to later stages. For example, mam- 

 malian embryos, which have many early characteristics in common with 

 those of fish, amphibia and reptiles, have other structures which enable 

 them to survive and develop within the mother's uterus rather than 

 within an egg shell. Such secondary traits may alter the original char- 

 acters common to high and low forms so that the basic resemblances are 

 blurred. The concept of recapitulation must be used with caution, rather 

 than rigorously, but it does provide an explanation for many otherwise 

 inexplicable events in development. 



Studies of the embryonic forms may provide the only means for 

 identifying the relationships of certain organisms. Sacculina, for example, 



