94 The Source of Variability 



to the three-shield condition in Ecnomus. This array of circumstan- 

 tial evidence points strongly to the occurrence in each of these 

 four lines, of parallel, homologous, homeotic mutations, bringing 

 the dorsal region of the second and third thoracic segments under 

 the same developmental control as the first. 



Concerning horse evolution Robb (1936, 1937) and Simpson 

 (1944) thought that the change from a three-toed horse to a one- 

 toed horse (the two side toes represented by splints) occurred in 

 one stage and in one step, due to a change in the proportionate 

 development of the toes. This would imply the possibility of a 

 single mutation causing drastic change, but it is possible that a 

 series of progressive mutations were involved ( Simpson, 1953 ) . 



Another type of large mutation is the duplication of parts. Oc- 

 casionally a specimen shows a striking duplication such as the flea 

 having an extra pair of legs (Fig. 22), reported by Sanjean and 

 Travis ( 1955 ) . This condition may have been caused by a somatic 

 mutation. Less striking but heritable is Polydactyly in humans, in 

 which a single mutation causes the production of an extra finger 

 or toe. 



Taken together, these examples and circumstances indicate that, 

 although small character mutations may be the rule, some much 

 larger character mutations do occur and become established in their 

 respective populations. It is probable that, as far as chemical re- 

 organization of the genie material is concerned, these mutations 

 producing larger phenotypic changes are no more complex geneti- 

 cally than those producing small changes. 



THE ORDER IN GENETIC CHANGE 



The observed continuity of mutation in experimental populations, 

 the occurrence of similar mutations in nature, and the diversity of 

 undoubtedly genetically-controlled characters in organisms back 

 to the earlier fossil records, together tell a story of constant genetic 

 change since life began. This change has not been random but, 

 like pre-biological change before it, has moved in the direction of 

 a kind of order peculiar to itself. 



The order in pre-biological chemical evolution was determined 

 by temperatures, the possible kinds of chemicals mixed together, 

 and the inherent chemical and physical properties of the resultant 

 units. 



The order in genetic change also is restricted in one direction by 

 its chemical nature in that only certain new bondings or breaks can 



