562 The American Naturalist. [July, 
plasm lower in degree than the visible cell-units, to the inter- 
nal or polar forces of which and their modification by exter- 
nal agencies and interaction, he ascribes the ultimate responsi- 
bility in reproduction, heredity and adaptation. This idea of 
biological units seems to me an essential part of any theory ; 
it is embodied in Darwin’s “gemmules,” in Haeckel’s “ plas- 
tidules,” yet, as Lankester says, the rapid accumulation of bulk 
is a theoretical difficulty in the material conception of units. 
In the direction of establishing some analogy between the rep- 
etition power of heredity and known function of protoplasm, 
Haeckel' and Hering” have likened heredity to memory, and 
advanced the hypothesis of persistence of certain undulatory 
movements; the undulations being susceptible of change and 
therefore of producing variability, while their tendency to per- 
sist in their established harmony is the basis of heredity. 
Berthold, Gautier and Geddes? have speculated in the elabora- 
tion of the idea of metabolism; the former holding the view 
that “inheritance is possible only upon the basis of the funda- 
mental fact that in the chemical processes of the organism the | 
same substances and mixtures of substances are reproduced in 
quantity and quality with regular periodicity.” 
I have merely touched upon these speculations to show that 
the unknown factors in heredity are also the unknown factors 
in operation in living matter. All we can study is the exter- 
nal form and conjecture that this form represents matter 
arranged in a certain way by forces peculiar to the organism. 
These forces are exhibited or patent in the somatic cells; they 
are potential or latent in the germ-cells. 
The last stage of our inquiry is as to the mode in which the 
action of habit or environment upon the somatic cells can be 
brought to bear upon the germ-cells. 
1Perigenesis der Plastidule oder die Wellenzeugung der Lebenstheilchen. Jena, 
1875. 
"Ueber d. Gedichtniss als eine allgemeine Function d. organischen Materie. 
Vienna, 1870. 
3See also Thomson, op. cit., p. 102. 
‘Berthold : Studien über Protoplasma-Mechanik. Leipzig, 1886. 
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