THEORIES OF HEREDITY 53 



vidual. According to this theory, therefore, the germ- 

 cells, and with them the new progeny arising out of them, 

 are permanently modified by environmental forces acting 

 on the parent-body, or, in other words, characters acquired 

 by the parents are, as a matter of course, inherited by the 

 offspring. 



For this purpose Spencer had to assume, as we shall 

 find Darwin similarly did, that the physiological units 

 circulate through the body, and " that in course of time 

 all of them visit all parts of the organism." 



Great as is the conception of physiological units by 

 Spencer — having served as the starting-point for nearly 

 every modern theory of heredity — there are three out- 

 standing criticisms which have to be made against this 

 theory : Firstly, the simple quality of organic polarity of 

 his- units is not sufficient to explain the manifold formations 

 of the organic body, least of all the wonderful changes 

 during the progressive stages of embryogenesis. Indeed, 

 to say that the units have a natural proclivity towards the 

 formation of the body amounts hardly to much more than 

 a verbal explanation. Secondly, the theory assumes the 

 germ-cells to be nothing more than small groups of physio- 

 logical units, being, as it were, an offshoot of the body. 

 As will be shown later, this is not consistent with the now 

 generally accepted idea of the relationship between body- 

 and germ-cells. Spencer asserts that the germ-cells are 

 in no essential different from any other body-cell, holding 

 that every body-cell would, under ideal conditions, be 

 capable of reproducing the whole body (refer to Begonia 

 leaf) : a generalization hardly supported by facts. And, 

 lastly, Spencer tries to deduce from the general principles 

 of force that modifications wrought on the parent-bod}^ 

 must necessarily produce the same modification in the 

 offspring, an inference which cannot legitimately be made 

 for biological phenomena, leading, as it does, to the un- 

 warranted, experimentally disproved assumption that the 

 physiological units actually circulate through the body. 



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