The Consequences of Evolution - 555 



CARPAL 



HUMERUS-/ / HUMERUS 



HUMERUS 



Fig. 29-1. Homologous bones of the forelimbs of various vertebrates. 



in some ancestral species. Or to state the 

 matter more specifically, the vestigial organ 

 is the product of a surviving group of genes 

 derived from genes that previously gov- 

 erned the development of some useful organ 

 in an ancestral species. As a general rule, 

 organs that become useless to a species as 

 a result of changes in its habits and environ- 

 ment tend, in the course of evolution, to 



degenerate in size and refinement of struc- 

 ture. The eyes, for example, of many cave- 

 dwelling species of crustaceans, fishes, and 

 amphibians, are blind and degenerate, al- 

 though these nonfunctional eyes still show 

 a similarity of structure and development to 

 the functional eyes of closely related day- 

 light-dwelling species. Given adequate time, 

 such an evolutionary degeneration of useless 



