60 



The Source of Variability 



"sport" appears as well, a variant of greater degree, such as a 

 black sheep among the white, a red-eyed fly among a black-eyed 

 species, or an eight-legged flea among the normal six-legged indi- 

 viduals (Fig. 23). Experimentation shows that many of these vari- 



(a) 



Fig. 23. An eight-legged flea (fleas normally have only six legs), (a) photo- 

 graph showing all eight legs of the flea, (b) Drawing ■ howing the four legs 

 on one side in proper relation to each other. (After Sanjean and Travis.) 



ants breed true and represent inherited changes in the genetic make- 

 up of the individual. These genetic variants, both large and small, 

 resulting from a wide variety of causes, are in essence the building 

 blocks of evolution. 



Fossils record the fact that various kinds of life have changed 

 radically from eon to eon, attesting the continuous origin of new 

 genetic variants since the record began. The great diversity of life 

 which had already evolved when the existing record began, now 

 some 600 million years of age, is excellent testimony that new 

 genetic variants have been arising ever since life began. This leads 

 to the questions: What caused the variants? How did these variants 

 accumulate and bring about changes in the kinds of life? Did every- 

 thing change gradually and methodically at the same rate, resulting 

 in a sort of orderly and predictable flow of new types, or was the 

 process of change irregular and unpredictable? 



Examining the geological record in regard to these questions 

 specifically, we find that in the past certain phylogenetic lines 

 evolved into highly- specialized creatures, such as the dinosaurs, 



