CHAPTER XI 



INHERITANCE 



Before proceeding to a consideration of the subsequent 

 history of the fertiUsed egg it is necessary to take into account 

 an aspect of fertilisation which is significant to the final result 

 of the process. The entry of the sperm into the egg not 

 only provides the stimulus for further development, but 

 influences the development so that the new individual bears 

 resemblance to the male as well as to the female parent. The 

 study of reflex phenomena has provided an instance of a field 

 of physiological inquiry which has been made susceptible to 

 quantitative treatment by the work of Sherrington, Pavlov, 

 and others, though the physicochemical basis of the phenomena 

 themselves is but little understood. The ultimate mechanics 

 of hereditary transmission is perhaps even more obscure ; 

 but there is hardly any branch of biological research which 

 has attained a higher degree of precision in the quantitative 

 treatment of those relations with which it is concerned. The 

 excellent presentation of the existing state of knowledge in 

 such recent works as that of Crew is sufficient excuse for 

 omitting a large mass of experimental detail. To such 

 the reader may turn for confirmation of statements which, 

 owing to lack of space for detailed treatment, may appear to 

 be dogmatic. One can, however, hardly omit all reference 

 in an account of this nature to properties common to all 

 organisms, and properties concerning which, moreover, the 

 bulk of our knowledge is derived from the study of the lower 

 organisms. 



The Factorial Kypothesis.— The exact study of inheritance 

 begins in the opening years of the present century with the 

 rediscovery by Tschermak, Correns and de Vries of certain 

 principles originally formulated by a contemporary of Darwin, 



