SOME CONSEQUENCES OF EVOLUTIONARY RELATIONSHIP 



367 



the gene changes and additions that have been incorporated into its gene 

 complex. 



Recapitulation is incomplete and distorted. Embryos are never 

 replicas of ancestral forms but merely show a general resemblance to 

 them. They often fail to show features possessed by their known ancestors 

 and, as a rule, can scarcely represent any ancestral stage except in barest 

 essentials. Ontogeny does not repeat phylogeny in detail. 



It is easy to see why this must be so. Changes in the gene complex may 

 affect the organism at any stage from germ cell to adult, and some or 

 many are almost certain to affect embryonic processes. Furthermore, 

 embryos have to live, just as do adult organisms. They must be adapted 



Fig. 24.9. A larval insect highly modified for predatory aquatic life, shown seizing a mos- 

 quito larva. The adult is the familiar winged dragonfly (Odonata). The enormous enlarge- 

 ment of the lower lip and its adaptations for capturing prey are larval specializations added 

 to the life history. (Courtesy American Museum of Natural History.) 



to their environments if they are to survive and hence are subject to 

 selection and evolutionary modification. The adjustments that embryos 

 show fall into two principal classes — changes in the rate of processes, and 

 adaptations for life in special environments. 



Embryonic development must be completed in a matter of hours, days, 

 or months, and yet it must accomplish the changes that are the result of 

 millions of years of evolution. The developmental processes therefore 

 tend to be " telescoped," partly by omission of stages, partly by the 

 overlapping of processes which in phylogeny were successive. Such modi- 

 fications tend to alter the historical sequence of changes, so that the 

 embryonic record is not only condensed but often full of anachronisms. 

 Thus in a vertebrate embryo the eye begins to form very early, far out of 

 chronologic sequence, following the lead of the precocious brain. 



Many embryos have developed special structures that enable them to 

 feed upon stored food, to respire, and to dispose of metabolic wastes 



