EVOLUTION 355 



If it is clearly realized that, in discovering the causes of evolution, it is 

 only the origin of mutations that is being considered, the discussion will 

 be simplified. It matters not, in this connection, what kind of mutation 

 arises, nor whether the one or several animals which possess the new 

 character survive or perish. These are questions which are discussed 

 below under the caption "Course of Evolution." Just now the thing 

 requiring explanation is the origin of heritable changes. 



Internal Factors: Mutations. — It has been indicated elsewhere that 

 the characters of an animal are determined by something in the chromo- 

 somes of the germ cells from which the animal develops. Although 

 individual characters may be impressed upon an organism by the environ- 

 ment, such features are rarely if ever, in the higher animals, handed on 

 to the offspring. An evolutionary change must therefore presumably 

 begin with a change in the chromosomes of the germ cells. What kind 

 of change this must be is uncertain, for it is not clear what kinds of 

 substances in the chromosomes are responsible for the development of 

 hereditary traits. If the proteins determine development and heredity, 

 a mutation may well result from a rearrangement of the atoms in the 

 molecules. Proteins are very complex substances, their molecules being 

 composed of hundreds or even thousands of atoms. No doubt, as in 

 simpler organic compounds, these molecules have a definite configura- 

 tion; that is, the proteins possess structure even when the chemist can- 

 not discover it. In such complex bodies, by slight modifications, new 

 molecules could be produced in which exactly the same atoms were 

 present, but in different arrangement. Two or more substances whose 

 molecules have the same number of atoms of each of the same elements, 

 but differently arranged, are know^n as isomers. Isomers are known in 

 many organic compounds, and they invariably differ from one another 

 in some physical or chemical property. A substance may often be 

 easily converted into one of its isomers, as for example, by the appHcation 

 of heat. If the proteins of the chromosomes should suffer any rearrange- 

 ment of their parts, and their chemical properties be thereby altered, 

 any hereditary traits determined by the modified proteins could hardly 

 escape being different. Moreover, if the rearranged molecules were fairly 

 stable, they would remain in the changed condition in succeeding genera- 

 tions, and the trait which they produced would be inherited. The 

 altered condition would therefore be a mutation, a step in evolution. 

 What sort of change in the adult would follow a given change in the 

 proteins in the chromosomes could not be predicted. A shift of a few 

 atoms to a new position, in the germ cells, might change the color of the 

 eye or the shape of a bone. It is extremely unhkely that a molecular 

 change of the kind hypothecated would not produce any alteration what- 

 ever in the adult. In view of the uncertainty regarding the nature 

 of the material basis of heredity, further speculation of this kind would 



