Spencer's Physiological Units . ' 51 



We have now critically to compare these various 

 opinions. In doing so the chief question is in how far 

 the hypotheses themselves, as they have just been de- 

 scribed, and without further ancillary hypotheses, can 

 lead to an explanation of the phenomena of heredity. 



5. Spencer's Physiological Units 



In his famous system of Synthetic Philosophy, Her- 

 bert Spencer attempted, probably for the first time, to 

 formulate a material conception of heredity. His Prin- 

 ciples of Biology, which form the second and third volume 

 of that system, appeared in 1864 and 1867, therefore 

 before the publication of Darwin's pangenesis (1868). 

 His train of thought is essentially as follows : 



Bud-formation from leaves, et cetera, teaches us that 

 the living particles of these organs possess the power of 

 reproduction, which is also shown in animals by the res- 

 toration of lost members. Now these particles cannot be 

 the cells themselves, because some cells can also replace lost 

 parts. Just as little can they be chemical molecules, be- 

 cause these are much too simply constructed for an ex- 

 planation of all the morphological differences. They 

 must, therefore, be units of intermediate size, invisibly 

 small, but composed of numerous molecules. Spencer 20 

 calls them physiological units. 



Every one of these units represents the entire specific 

 character; slight dissimilarities in their structure cause 

 the differences between allied species (p. 183). 



Spencer finds it difficult to explain fertilization. 

 There is no sense in it unless there is some kind of dif- 

 ference between the two groups of physiological units. 



20 Spencer, H. Principles of Biology. Ed. 2. 1: 180-183. 



