SUPPLEMENT III 



THE PRODUCTION OF MUTATIONS 



IF. as Darwin maintained, the adap- 

 tiveness of living things results from 

 natural selection, rather than from a 

 teleological tendency in the process of 

 variation itself, then heritable variations 

 must, under most conditions, occur in 

 numerous directions, so as to give a wide 

 range of choice for the selective process. 

 Such a state of affairs seems, however, 

 in more or less contradiction to the com- 

 monly held idea, to which Darwin also 

 gave some credence, that heritable varia- 

 tions of given kinds tend to be produced, 

 in a fairly regular way. by given kinds 

 of external conditions. For then we are 

 again confronted with the difficulty, how 

 is it that the "right kinds" of variations 

 (i.e. the adaptive ones) manage to arise 

 in response to the "right kinds" of con- 

 ditions (i.e. those they are adapted to) ? 

 Moreover, the de Yriesian notion of mu- 

 tations does not help us in this connec- 

 tion. On that view, there are sudden 

 jumps, going all the way from one "ele- 

 mentary species" to another, and involv- 

 ing radical changes in numerous charac- 

 ters at once, and there are relatively few 

 different jumps to choose between. This 

 obviously would fail to explain how, 

 through such coarse steps, the body 

 could have come to be so remarkably 

 streamlined in its internal and external 

 organization, or, in other words, so 

 thoroughly adaptive. 



The older selectionists, thinking in 

 terms of chemical reactions on a molar 

 scale when they thought in terms of 

 chemistry at all, did not realize suffi- 

 ciently the ultramicroscopic random- 

 ness of the processes causing inherited 

 variations. The earliest mutationists 

 failed, in addition, to appreciate the 

 qualitative and quantitative multiplicity 

 of mutations. It was not long, however, 

 before the results of Baur on Antirrhi- 



num and of Morgan on Drosophila. sup- 

 plemented by scattered observations on 

 other forms, gave evidence of the occur- 

 rence of numerous Mendelizing muta- 

 tions, many of them small ones, in varied 

 directions, and they showed no discover- 

 erable relation between the type of mu- 

 tation and the type of environment or 

 condition of living under which it arose. 

 These observations, then, came closer to 

 the statistical requirements for a process 

 of evolution which has its basis in acci- 

 dents. In what sense, however, could 

 the events be regarded as accidental ? 

 Were they perhaps expressions of veiled 

 forces working in a more determinate 

 manner? It was more than ever evident 

 that further investigation of the manner 

 of occurrence of mutations was called 

 for. 



If the mutations were really non-teleo- 

 logical. with no relation between type of 

 environment and type of change, and 

 above all no adaptive relation, and if 

 they were of as numerous types as the 

 theory of natural selection would de- 

 mand, then the great majority of the 

 changes should be harmful in their ef- 

 fects, just as any alterations made blind- 

 ly in a complicated apparatus are usually 

 detrimental to its proper functioning, 

 and many of the larger changes should 

 even be totally incompatible with the 

 functioning of the whole, or, as we say. 

 lethal. That is. strange as it may seem 

 at first sight, we should expect most mu- 

 tations to be disadvantageous if the the- 

 ory of natural selection is correct. We 

 should also expect these mainly disad- 

 vantageous changes to be highly diversi- 

 fied in their genetic basis. 



Frequency of Mutations 



To get exact evidence on these points 

 required the elaboration of special genet- 



s-19 



