EVOLUTION 13 



some genetic differentiation during embryonic develop- 

 ment. 



Though no knowledge of either genes or chromosomes 

 was available in Darwin's day, it was known to him, and 

 to plant and animal breeders generally, that occasionally 

 a carefully inbred stock produced a 'sport' differing mark- 

 edly from the parents in one or another specific charac- 

 ter, and that the new character was stable in that it was 

 inheritable. This sudden and random appearance of new 

 varieties was first called mutation' by the Dutch botanist, 

 Hugo De Vries, in 1901, who had studied it carefully in 

 the American evening primrose (Oenothera lamarcki- 

 ana) . De Vries believed that it was such sudden changes 

 that gave rise to new species and afforded the raw mate- 

 rials of evolution, but it was subsequently shown that 

 his mutations among primroses were the result simply 

 of 'chromosome mutations,' which may take the form of 

 a change in the number of chromosomes (a phenomenon 

 that occurs more frequently in plants than in animals), 

 of a rearrangement of genes within a chromosome, or a 

 transfer of a gene from one chromosome to another. If 

 De Vries was wrong about the basic nature of his prim- 

 rose mutations, he was right in principle in surmising 

 that sudden— or as he called them, 'explosive'— changes 

 can occur in the basic determiners of hereditary charac- 

 ters, the genes; and his term 'mutation' is appHed today 

 not only to chromosomal mutations but also to sudden 

 transformations in the genes themselves. Such genie mu- 

 tations produce new body characters which, so long as 

 the new gene remains stable, are inherited according to 

 the laws of genetics. Genie mutations have been inten- 

 sively studied in many species of animals and plants and, 

 along with chromosomal mutations, are conceived to 

 supply the 'variations' which afford the raw materials of 

 evolution. 



Mutations are said to be 'spontaneous,' in that they 

 occur independently of all known environmental influ- 

 ences. It is known that high energy radiation of cells in- 



