IN THE THEORY OF NATURAL SELECTION. 271 



In the first place it may be argued that external influences may 

 not only act on the mature individual, or during- its development, 

 but that they may also act at a still earlier period upon the germ- 

 cell from which it arises. It may be imagined that such influences 

 of different kinds might produce corresponding minute alterations 

 in the molecular structure of the germ-plasm, and as the latter is, 

 according to our supposition, transmitted from one generation to 

 another, it follows that such changes would be hereditary. 



Without altogether denying that such influences may directly 

 modify the germ-cells, I nevertheless believe that they have no 

 share in the production of hereditary individual characters. 



The germ-plasm or idioplasm of the germ-cell (if this latter term 

 be preferred) certainly possesses an exceedingly complex minute 

 structure, but it is nevertheless a substance of extreme stability, for it 

 absorbs nourishment and grows enormously without the least change 

 in its complex molecular structure. With Nageli we may indeed 

 safely affirm so much, although we are unable to acquire any direct 

 knowledge as to the constitution of germ-plasm. When we know 

 that many species have persisted unchanged for thousands of years, 

 we have before us the proof that their germ-plasm has preserved 

 exactly the same molecular structure during the whole period. I 

 may remind the reader that many of the embalmed bodies of the 

 sacred Egyptian animals must be four thousand years old, and that 

 the species are identical with those now existing in the same 

 locality. Now, since the quantity of germ-plasm contained in a 

 single germ-cell must be very minute, and since only a very small 

 fraction can remain unchanged when the germ-cell developes into 

 an organism, it follows that an enormous growth of this small 

 fraction must take place in every individual, for it must be re- 

 membered that each individual produces thousands of germ-cells. 

 It is therefore not too much to say that, during a period of four 

 thousand years, the growth of the germ-plasm in the Egyptian ibis 

 or crocodile must have been quite stupendous. But in the animals 

 and plants which inhabit the Alps and the far north, we have 

 instances of species which have remained unchanged for a much 

 longer period, viz. for the time which has elapsed between the close 

 of the glacial epoch and the present day. In such organisms the 

 growth of the germ-plasm must therefore have been still greater. 



If nevertheless the molecular structure of the germ-plasm has 



