102 ON HEREDITY. 



Crustacea is apparently colourless, but the microscope reveals the 

 presence of a number of beautiful pigment spots ; and not until 

 these have increased enormously does the skin appear coloured 

 to the naked eye. The presence or absence of colour and its 

 quality when present are here dependent upon the quantity of 

 the most minute particles, and on the distance at which the 

 object in question is observed. Again, the first appearance of 

 colour, or the change from a green to a yellow or red colour 

 depends upon slight variations in the position or in the number 

 of the oxygen atoms which enter into the chemical combination 

 in question. Fluctuations in the chemical composition of the mole- 

 cules of a unicellular organism (for example) must continually 

 arise, just as fluctuations are always occurring in the number of 

 pigment granules in a certain cell, or in the number of pigment 

 cells in a certain region of the body, or even in the size of the 

 various parts of the body. 



All these quantitative relations are exposed to individual fluctua- 

 tions in every species ; and natural selection can strengthen the 

 fluctuations of any part, and thus cause it to develope further in 

 any given direction. 



From this point of view, it becomes less astonishing and less 

 inconceivable that organisms adapt themselves as we see that they 

 obviously do in all their parts to any condition of existence, and 

 that they behave like a plastic mass which can be moulded into 

 almost any imaginable form in the course of time. 



If we ask in what lies the cause of this variability, the answer 

 must undoubtedly be that it lies in the germ-cells. From the 

 moment when the phenomena which precede segmentation com- 

 mence in the egg, the exact kind of organism which will be 

 developed is already determined whether it will be larger or 

 smaller, more like its father or its mother, which of its parts will 

 resemble the one and which the other, even to the minutest detail. 

 In spite of this, there still remains a certain scope for the influence 

 of external conditions upon the organism. But this scope is 

 limited, and forms but a small area round the fixed central point 

 which is determined by heredity. Abundant nourishment can 

 make the body large and strong, but can never make a giant out 

 of the germ-cell destined to become a dwarf. Unhealthy seden- 

 tary habits or insufficient nourishment makes the i'adory-luind pale 



