206 LECTURES ON BIOLOGY 



This universal rule called, after its discoverer, Quetelet's law- 

 does not only apply to differences in size but apparently also 

 to all other variations. We have already heard of the case of 

 the vertebrae of the herring, which also follow Quetelet's law. 



While the variability remains generally unchanged as long as 

 circumstances remain unchanged, it is possible to alter the average 

 by a selection of greatly differing variations. But the longer the 

 selection operates the less progress is made, until finally a point 

 is reached which apparently cannot be passed by selection. Let 

 us consider in this light the breeding experiments made by 

 Miiller with maize. His attention was directed to the number 

 of rows in the cobs of Zea mays, which in the beginning of the 

 experiment varied between eight and eighteen. Twelve occurred 

 most frequently. If he sowed corns taken from a cob with 

 sixteen rows he obtained descendants having from ten to twenty- 

 two rows ; the average, therefore, had been shifted to fourteen. 

 In proportion as the experiment was continued the average was 

 gradually shifted to eighteen. At this point it came to a stand- 

 still. Strict selection was able to maintain this average but 

 unable to increase it, while in the absence of selection the 

 descendants returned to the original average of twelve. The 

 characteristics remained, therefore, constant only so long as 

 selection was maintained, and could only be increased up to a 

 certain point, beyond which selection was powerless. Whether, 

 in the event of the experiments being continued for very long 

 periods, a fixing of the characters obtained by selection and 

 a further shifting of the average is possible must at present 

 be regarded as an open question. 



In addition to the fluctuating or individual variability we 

 may occasionally observe a jump in variations, described as 

 ' mutation.' The most important difference between these two 

 forms is that the qualities produced by mutation are strictly 

 hereditary. But mutation frequently affects not only one or 

 another character ; it changes also more or less suddenly the 

 entire character of the affected organism. While in former 

 years mutations were not thought to be of any importance to 

 the origin of new species, De Vries made them the foundation 

 of a new theory of evolution. The classic object with which this 

 great investigator conducted his experiments was (Enothera 



