268 GENETICS IN RELATION TO AGRICULTURE 



its own distinctive manifestation in one-fourth of the progeny of such 

 individuals. 



When we enquire as to what are the particular conditions or specific 

 antecedent events that make possible or cause the assumed substitution 

 of atoms or radicals, we find ourselves again confronted by an almost 

 total lack of knowledge. One thing is certain however, namely, that 

 factor mutations are not fortuitous in occurrence, because, if they were 

 the outcome of wholly indeterminate series of events, they would be 

 as likely to occur in one species or race as in another at a given time 

 and with the same relative frequency under all conditions, but such is 

 not the case. On the contrary, certain species appear to be much more 

 prolific in factor mutations than others and, as stated in Chapter II, 

 it would appear that inheritable variations can be induced under 

 controlled environmental conditions in pedigree strains that have 

 bred true for a number of generations. Furthermore, even though 

 our knowledge of the occurrence of factor mutations were so meager 

 as to furnish no basis for reasoning and even though future observations 

 of the same might seem to indicate that they are fortuitous, we should 

 still be justified in assuming the existence of specific causes for factor 

 mutations. It has been clearly shown by Pearl that, while natural 

 phenomena are the result of long series of antecedent events or con- 

 ditions, yet these are not all of equal determinative value; but rather 

 that there are always specific causes which are few in number, immediate 

 in time and large in relative quantitative effect. It does not seem 

 necessary to present here the course of reasoning on which this con- 

 clusion rests. The important thing for agriculture is the fact that 

 factor mutations are caused and the possibility that some of the deter- 

 minative antecedent conditions are external to organisms, i.e., that 

 they exist in the environment and are controllable by man. The prob- 

 lem of the exact nature of factor mutations is only a phase of the 

 general problem of the nature of living protoplasm, the solution of which 

 is one of the ultimate aims of biology. But it is possible at least that 

 experimental research may reveal methods by which factor mutations 

 can be induced in both plants and animals. 



Factor Mutations Both Germinal and Somatic. Factor mutations 

 appear to occur in undifferentiated cells, the germ plasm or embry- 

 onic tissue in animals and either the germ cells or any meristematic 

 tissue in plants. Occasional discontinuous variations are found in 

 animals which might seem at first to be due to factor mutations in the 

 developing soma. But most of these abnormalities are more satis- 

 factorily explained in other ways. Thus, gynandromorphism, or the 

 condition of having one side of the body male and the other female, 

 has been reported in insects more than a thousand times according to 



