INTRODUCTION 



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A. — Historical Part 



ERBERT SPENCER was practically the first in the 

 present generation to attempt a theoretical explanation 

 of heredity when he propounded his theory of ' physiolog- 

 ical units.' The regeneration of lost parts, e.g., of a leg or the 

 tail of a salamander, led him to the conception of these units, 

 ' in all of which there dwells the intrinsic aptitude to aggregate 

 into the form of that species ; just as in the atoms of a salt 

 there dwells the intrinsic aptitude to crystallise in a particular 

 way.'' He calls this aptitude the ' polarity of the organic units,' 

 and defines the latter as being intermediate between the 

 ' chemical units ' or molecules and the ' morphological units ' 

 or cells. They must be 'immensely more complex than the 

 chemical units,' and must therefore correspond to groups of 

 molecules. It is very interesting at the present day, now that 

 we have advanced somewhat further towards a theory of 

 heredity, to summarise the various aptitudes and forces which 

 Herbert Spencer thought it necessary to ascribe to his ' physio- 

 logical units,' in order to arrive at an explanation of the 

 phenomena. Although the sections on Heredity and Regener- 

 ation constitute only a small portion of his great work on the 

 ' Principles of Biology,' and cannot therefore contain a detailed 

 treatment of the phenomena of heredity, his opinions on this 

 subject are evident. 



Spencer considers, on the one hand, that the whole organism 

 is composed of these units, which are all alike in kind, and on 

 the other, that the germ-cells also contain small groups of them. 

 The former supposition makes regeneration possible to each 

 sufficiently large portion of the body, while the latter gives the 

 germ-cell the power of reproducing the whole: inasmuch as the 



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