308 DARWINISM TO-DAY. 



not influence them? They will die if removed from the water, and 

 develop abnormally if certain substances be added to or subtracted 

 from the water. But the embryonic differentiation is a result of 

 chromosomal activities, as we have seen ; therefore, changes in the 

 medium must influence the germ-plasm. Is not a well-recognised 

 characteristic of living matter, response to external stimuli ? How 

 can any living cell be acted upon by stimuli and yet not react to 

 them? There is not a single observation to show that any germ- 

 cell, or its germ-plasm, is in any way guarded or protected from 

 external stimuli, either by the structure of its cell wall or peripheral 

 cytoplasm, or by the nature of the living tissue that encloses it. 

 And whatever affects the cell body must indirectly affect the 

 chromosomes, because of the physiological connection of the two. 



"From such considerations it would seem practically certain that 

 the energies of the chromosomes are to some extent respondent to 

 environmental stimuli. And since observation shows that living 

 matter responds differently, if not always according to the nature 

 of the stimulus at least to its degree, it would follow that change in 

 the nature or degree of the environmental agencies would indirectly 

 engender change in the activities of the germ-plasm. Not to admit 

 this would be to deny to the germ-plasm, without empirical reason, 

 properties proven for all other living substances. 



"This thought had long ago been expressed clearly, though from 

 a different line of reasoning, by Spencer (1865). We may state it 

 in another way. Tissue-cells are granted by experimental physiolo- 

 gists the ability of different response, or different intensity of 

 response, to stimuli of different kind or degree. But a tissue-cell 

 is a lineal descendant of a germ-cell, and receives germ-plasm 

 from the latter. Now since the germ-plasm has been transmitted 

 continuously to the tissue-cell, must not the energies of the germ- 

 plasms of the two be alike at least in their general response 

 activity? Again, a Protozoan may be considered as a cell not 

 exactly correspondent to a germ-cell of a Metazoan, but as some- 

 thing more, as a unit with properties of both a germ-cell and a 

 tissue-cell, for its cytoplasmic differentiations (cilia, contractile 

 vacuoles, cytopharynx, etc.) are comparable to the soma of a Meta- 

 zoan. In the case of the Protozoan Paramos cium, Calkins (1904) 

 has shown that the reproductive activity is increased or diminished 

 according to the amount and kind of food stimuli. Here, then, a 

 Protozoan has its reproductive activities, therefore the energies of 

 its germ-plasm, profoundly influenced by environmental changes; 

 and it is primarily what we may term the germ-cell constituent of 

 the Protozoan that becomes influenced, that part which has to do 

 with reproduction of the individual. Weismann considered the 



