MODIFICATION OF SPECIES 357 



animals appears to offer no possible mechanism whereby an altered body 

 can produce in the germ cells within it any modification such that 

 offspring developing from them would have the same alteration. More- 

 over, though many experimental attempts to produce such changes 

 have been made, no satisfactory evidence of their success has ever been 

 adduced. 



It seems necessary, then, to exclude somatic influence from the list of 

 possible causes of mutation. When mutations began to arise under 

 observation in experimental cultures, it was further observed that there 

 Avas no apparent difference between the environment of the one mutant 

 individual and all the rest. It was long supposed, therefore, that the 

 cause of mutation was an unknoAvn something within the animal, possibly 

 in some way connected with its physiological processes. In recent times, 

 however, it has been found that certain environmental agents are not 

 stopped by the body but reach the germ cells directly. They may not 

 influence the body in any detectable way yet produce modifications in the 

 germ cells. The most potent of these kno\\Ti agents is X rays, and the 

 most responsive organism is Drosophila. Hundreds of alterations have 

 appeared in the offspring when the parents were exposed to the rays. 

 Some of the modifications are visible structural changes; more of them 

 have physiological effects. How much natural mutation may be due to 

 such radiation is in doubt. Though there is always a certain amount of 

 radiation from the earth, it appears much too feeble to account for the 

 mutation that has occurred in laboratory cultures. Heat is the principal 

 other agent that has been found to produce mutations, and again Droso- 

 phila is the subject. While some parts of the earth have as high tempera- 

 ture as was employed in these experiments, the temperate zones, where 

 most of the thousand mutations of Drosophila have arisen in laboratories, 

 are not among them. 



On the whole, while it must now be recognized that external agents 

 may produce mutations b}^ direct action on the germ cells, the chief 

 agents have not yet been discovered; and the possibility of wholly 

 internal agents has not been exhausted. 



Hybridization. — Given a number of genes in which various members of 

 a species are different, an important other source of variation is at hand. 

 If individuals having different genes are capable of crossing, as they 

 nearly always are ^dthin a single species, the genes may be combined in 

 different ways. How many recombinations may be produced depends 

 only on the contrasting genes. If there are only 20 spots in the chromo- 

 somes at each of which, somewhere in the population, two different genes 

 exist, it is possible to have over a million different kinds of individuals. 

 Most species presumably have more than 20 mutant genes floating about 

 in scattered members, and for each additional mutation the number of 



