490 Genetic Theory and Evolution 



a necessary partner of genetic change in the understanding of evolu- 

 tion. 



Demonstrating the possibihty of saltations in principle means, of 

 course, to show that developmental processes can be shifted by one 

 mutant to such an extent that a structural departure appears which is 

 of the order of magnitude of macroevolutionary differences. This must 

 mean, in terms of development, that the mutant action affects one of 

 the early determinative processes which decide the fate of a develop- 

 ing organism. We discussed such processes earlier (III 5 D e). A good 

 example which may serve as a model (see figs. 21, 22) for any others 

 is the Drosophila mutant tetraltera (one of the podoptera types). 

 Here it can be seen that definite sections of the wing anlage are 

 redetermined: one part into a leg or haltere-like structure by sepa- 

 rating it from the rest and transforming it into a whole by adding a 

 mirror image half; other parts of the wing anlage being redetermined 

 into dorsum and scutellum, with their typical bristles, and incorpo- 

 rated into the thorax. In the same example we have seen the very 

 important fact (figs. 21, 22) that the redetermined part subsequently 

 becomes subjected to the power of integrating new and old structures 

 in an appropriate way into a whole; this is one of the least under- 

 stood properties of animal development, but it is an undeniable fact 

 which is of the greatest importance for evolution. In the present 

 example the transformed wing anlage was fitted in a perfect way into 

 a completely reconstructed dorsal mesothorax, while the transformed 

 scutellum with the normal one was crowded out and disappeared in 

 the new architecture. This is a perfect example of how a macromutant 

 (or a few successive ones) can produce in one step a major structural 

 change, in itself perfect, integrated into the organism and changing 

 it in general order of magnitude at least (of course for the one 

 structure only in the observed example) to the extent of a generic 

 difference or even more. 



The number of such macromutants in animals and plants which 

 could serve as similar models is steadily increasing. Stubbe and 

 Wettstein (1941) and Stubbe (1952) described a number of them in 

 Antirrhinum flowers, all reminiscent of the type of different genera. 

 Stubbe showed also how they can easily acquire the necessary modifiers 

 (meaning readjustment of the genetic system) for normal vitality. 

 Burgeff (1943) found similar macromutants in liverworts; Andersson- 

 Kotto and Gairdner (1936) in ferns; Gustafsson (1947, 1951a) in 

 wheats; Lamprecht (1948) in peas; the most extreme case in plants 

 is probably that of corn grass (Singleton, 1951), in which not only a 



