PROBLEMS OF MODERN SCIENCE 



problem of heredity. A great deal of nonsense 

 has been talked about organisms being built up 

 entirely of separately heritable unit characters, 

 and about all species having arisen through the 

 crossing or hybridisation of pre-existing forms. 

 The truth appears to be that the Mendelian 

 factors, after all, are only modifying agents. 

 There are probably no special factors, for ex- 

 ample, that determine whether a normal human 

 being shall have a nose or no nose, but there 

 may be factors which determine whether his nose 

 shall be of one particular shape or another, just 

 as there appear to be factors which determine the 

 colour of his eyes. 



The common-sense view of heredity teaches 

 us that an organism resembles its parents because 

 it commences its existence with a certain stock of 

 body-forming substance similar to that with which 

 its parents commenced, and because this material 

 is subjected during its development to a series of 

 stimuli similar to those which influenced the 

 development of the parents. Like causes produce 

 like effects, and two series of developmental 

 events, starting with similar material and taking 

 place under similar conditions, must lead to 

 similar results. The offspring must resemble the 

 parent. If there be different modifying factors, 

 Mendelian or otherwise, present in the two cases, 

 the results must differ accordingly, and we know 



138 



