130 HEREDITY AND "ARRANGEMENT" 



tiny seed, it is so everyday that we have ceased 

 to wonder at it. It is there : we know that. 

 But have we any kind of idea how it comes about ? 

 The duck does not, as a matter of common ex- 

 perience, come out of a hen's egg. Why does it 

 come out of a duck's egg ? Why doesn't it come 

 out, if only rarely, from a hen's egg ? In other 

 words, do we know what it is that explains in- 

 heritance or how it is that there is such a thing 

 as inheritance ? Well, candour obliges me to say 

 that we do not. In spite of all the work which 

 has been expended upon this question we are 

 totally ignorant of the mechanism of heredity. 

 Nevertheless it will be instructive to glance at 

 the theories which have been put forward to 

 explain this matter. 



All living things spring from a small germ, and 

 in the vast majority of cases this germ is the 

 product in part of the male and in part of the 

 female parent. It is therefore natural that we 

 should in the first place turn our attention to this 

 germ and ask ourselves whether there is anything 

 in its construction which will give us the key 

 of the mystery. There is not, at least there is 

 nothing definite as shown by our most powerful 

 microscopes. To be sure there is a remarkable 

 substance, called chromatin because of its capacity 

 for taking up certain dyes, which evidently plays 

 some profoundly important part in the processes 

 of development. We may suspect that this is 

 the thing which carries the physical characteristics 

 from one generation to another, but we cannot 

 prove it ; and though some authorities think that 



