312 THE CONTINUITY OF THE RACE 



Autogenous variations. Another large assemblage of variations in- 

 cludes those known to be definitely inherited. Although many of these 

 cannot by inspection be distinguished from environmentally produced 

 variations, experiment shows that they are not due to a response to the 

 environment but must be caused by some internal change in the organism 

 itself. Such internal changes are spontaneous in the sense that they 

 are not produced by any known cause and can neither be predicted nor 

 duplicated by any effort of the experimenter. 1 Such variations first appear 

 in but one or a few 2 of the many individuals exposed to the same environ- 

 mental conditions; but once in existence the new variation may be passed 

 on to an increasing number of individuals that are descendants of the 

 original variant. 



Detailed studies of autogenous variations by the parallel methods 

 of the cytologist and the experimental breeder have shown that they are 

 not all of one type. Broadly speaking they fall into two main groups: 

 (1) variations that are correlated with various kinds of chromosome 

 change and that show various patterns of inheritance ; and (2) variations 

 that behave as units in Mendelian inheritance but that are not accom- 

 panied by any detectable change in the chromosomes. 



Autogenous Variations Due to Chromosomal Changes. We shall dis- 

 miss this group of inherited variations rather summarily. This is not 

 because they are rare or unimportant, but because at present they are of 

 more interest and importance to the technical geneticist and cytologist 

 than to the general student. It will be well to remember, however, that 

 some autogenous variations are caused by gross changes in the chromo- 

 somes — changes that affect several or many genes at once — and that 

 these changes provide valuable material for further studies of heredity 

 and cytological processes. We can characterize the main types of 

 known chromosomal changes associated with autogenous variations 

 as follows: 



1. Changes within a single chromosome including: loss of a small por- 

 tion of a chromosome, duplication of a portion of a chromosome, and 

 rearrangement of parts of a chromosome so that the normal linear se- 

 quence of a given block of genes is inverted. 



2. Exchanges of parts between different (nonhomologous) chromo- 



1 The fact that exposure of organisms to X rays, radium, and other agents does 

 very definitely increase the percentage of new inherited variations is not a real contra- 

 diction of this statement. Such agents do speed up the "mutation rate," but the 

 variations that appear are still unpredictable, are quite like those that appear at a 

 slower rate in the absence of radiation, and they have no discernible adaptation or 

 relation to the agent that increased their frequency. 



2 The finding of a new variation in several individuals simply means that the 

 variation was not detected until it had been inherited by the progeny of the original 

 variant. 



