H 



INTRODUCTION 



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 

 -y ' chemical units ' or molecules and the *• morphological units ' 

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

 jj^j chemical units,' and must therefore correspond to groups of 

 or"-' 'iiolecules. It is very interesting at the present day, now that 

 t]^„ we have advanced somewhat further towards a theory of 

 Ijjjy „ieredity, to summarise the various aptitudes and forces which 

 hjg ^erbert Spencer thought it necessary to ascribe to his ' physio- 

 loj „,gical units,' in order to arrive at an explanation of the 

 p] yi lenomena. Although the sections on Heredity and Regener- 

 ati,,^,„ [On constitute only a small portion of his great work on the 

 'Pi,- . rinciples of Biology,' and cannot therefore contain a detailed 

 tr/„ , eatment of the phenomena of heredity, his opinions on this 



si^? ^ ibject are evident, 

 ^restors 

 , " pencer considers, on the one hand, that the whole organism 



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



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



T,[. ? lie former supposition makes regeneration possible to each 



sij ifficiently large portion of the body, while the latter gives the 



gjv, ' arra-cell the power of reproducing the whole : inasmuch as the 



