350 



PRINCIPLES OF ANIMAL BIOLOGY 



recessive to the wild-type characters from which they sprang. Smaller 

 numbers of mutations liave been obser\^e(i to occur in other species of 

 flies, and in wasps among insects; in mice, rats, rabbits, and guinea pigs 

 among mammals; and in corn, barley, peas, and morning-glories among 

 plants. So freely have these and other organisms mutated that the bulk 

 of evolution may reasonably be assumed to follow from just such changes. 

 True mutations may be supplemented by the breakage or duplication of 

 chromosomes, but changes of this nature cannot be emphasized in an 

 elementary discussion. 



Causes of Mutation. — What causes mutations to occur under natural 

 conditions is still unknown. The genes are almost certainlj^ chemical, 

 and it is likely that they are fundamentally protein. If these surmises 

 are correct, mutations should be chemical modifications and of the sort 



Fig. 303. — Mutations in the vinegar flj' Drosophila melanogaster . A, normal wing; B, 

 beaded ■wing; C, notch wing; D, vestigial wing; E, miniature wing; F, club wing; G, rudi- 

 mentary wing; H, truncate wing; I, normal red eye; /, bar eye; K, eyeless; L, white eye. 

 (C from Morgan; D and L original; the rest from Morgan, Sturtevant, Muller, and Bridges, 

 courtesy of Henry Holt and Company, Inc.) 



that proteins are capable of undergoing. A century and a ciuarter ago 

 Lamarck (page 17), who was the first naturalist to propound a compre- 

 hensive theory of evolution, held that species changed in indirect response 

 to the environment, effected thi'ough use and disuse. Lamarck knew 

 nothing of the single character changes now called mutations; but, were 

 his idea correct, it would mean that mutations are caused by environ- 

 mental action. As Lamarck conceived the changes to occur, they con- 

 stituted inheritance of acquired characters. For the individuals were 

 supposed to be modified by the activity of the animals themselves which 

 led to such things as overdevelopment of the muscles or stretching of the 

 legs or neck. These changes were induced only in the soma or body at 

 first, but he believed that the body was then capable of influencing the 

 offspring in like manner. In the light of present knowledge, this influence 

 of the body on the offspring would have to take the form of causing 

 mutations in the germ cells while still in the body. It seems whollj^ 

 unlikely that any such influence can bo exerted. The organization of 



