MULLER 



proper development a nicely adjusted 

 train of processes, and so any change 

 in the genes— no matter whether loss, 

 gain, substitution or rearrangement— is 

 more likely to throw the develop- 

 mental mechanism out of gear, and 

 give a "weaker" result, than to inten- 

 sify it. For this reason, too, the most 

 frequent kind of mutation of all is the 

 lethal, which leads to the loss of the 

 entire organism, but we do not con- 

 clude from this that all the genes had 

 been lost at the time of the mutation. 

 The explanation for this tendency for 

 most changes to be degenerative, and 

 also for the fact that certain other 

 kinds of changes— like that from red to 

 pink eye in Drosophila—2LTe more fre- 

 quent than others— such as red to 

 brown or green eye— lies rather in de- 

 velopmental mechanics than in ge- 

 netics. It is because the developmental 

 processes are more unstable in one 

 direction than another, and easier to 

 push "downhill" than up, and so any 

 mutations that occur— no matter what 

 the gene change is like— are more apt 

 to have these effects than the other 

 effects. If now selection is removed in 

 regard to any particular character, 

 these character changes which occur 

 more readily must accumulate, giving 

 apparent orthogenesis, disappearance 

 of unused organs, of unused physio- 

 logical capabilities, and so forth. As 

 we. shall see later, however, the 

 changes are not so frequent or nu- 

 merous that they could ordinarily 

 push evolution in such a direction 

 against selection and against the im- 

 mediate interests of the organism. 



In regard to the magnitude of the 

 somatic effect produced by the gene 

 variation, the Drosophila results show 

 that there the smaller character 

 changes occur oftener than large ones. 

 The reason for this is again probably 

 to be found in developmental me- 

 chanics, owing to the fact that there 



111 



are usually more genes slightly affect- 

 ing a given character than those play- 

 ing an essential role in its formation. 

 The evidence proves that there are 

 still more genes whose change does 

 not affect the given character at all— 

 no matter what this character may be, 

 unless it is life itself— and this raises the 

 question as to how many mutations 

 are absolutely unnoticed, affecting no 

 character, or no detectable character, 

 to any appreciable extent at all. Cer- 

 tainly there must be many such muta- 

 tions, judging by the frequency with 

 which "modifying factors" arise, 

 which produce an effect only in the 

 presence of a special genetic complex 

 not ordinarily present. 



(b) The Localizatioji of the 

 Change 



Certain evidence concerning the 

 causation of mutations has also been 

 obtained by studying the relations of 

 their occurrence to one another. 

 Hitherto it has nearly always been 

 found that only one mutation has oc- 

 curred at a time, restricted to a single 

 gene in the cell. I must omit from con- 

 sideration here the two interesting 

 cases of deficiency, found by Bridges 

 and by Mohr, in each of which it 

 seems certain than an entire region of 

 a chromosome, with its whole cargo 

 of genes, changed or was lost, and also 

 a certain peculiar case, not yet cleared 

 up, which has recently been reported 

 by Nilsson-Ehle; these important cases 

 stand alone. Aside from them, there 

 are only two instances in which two 

 (or more) new mutant genes have 

 been proved to have been present in 

 the same gamete. Both of these are 

 cases in Drosophila— reported by 

 Muller and Altenburg (1921)— in 

 which a gamete contained two new 

 sex-linked lethals; two cases are not 

 a greater number than was to have 

 been expected from a random distri- 



