28 THE PHYSICAL BASIS OF INHERITANCE 



four-cell stage in the development of the lancelet. Marchal 

 describes a " legion of embryos " developing from a single ovum 

 of a peculiar Hymenopterous insect Encyrtus. In development, 

 indeed, a half may be as good as a whole. 



In reference to the difficulty raised in some minds by the 

 minuteness of the physical basis, it may be recalled that the 

 students of physics, who make theories regarding the sizes of 

 the atoms and molecules which they have invented, tell us that 

 the image of an ocean liner filled with framework as intricate 

 as that of the daintiest watches does not exaggerate the possi- 

 bilities of molecular complexity in a spermatozoon, whose actual 

 size is usually very much less than the smallest dot on the 

 watch's face. Secondly, as we learn from embryology that one 

 step conditions the next, and that one structure grows out of 

 another, there is no need to think of the microscopic germ-cells 

 as stocked with more than initiatives. Thirdly, we must re- 

 member that every development implies an interaction between 

 the growing organism and a complex environment without which 

 the inheritance would remain unexpressed, and that the full- 

 grown organism includes much that was not inherited at all, but 

 has been acquired as the result of nurture or external influence. 

 The fact is that size does not count for much in these matters, 

 and the difficulty that some beginners feel in believing that the 

 inheritance of the whale is packed into a pinhead-like egg is 

 mainly due to ignorance of what may be called the fine com- 

 plexity, or from another point of view the " coarse-grainedness," 

 which must form part of our conception of every speck of matter. 

 Nowhere more than in biology are we made to feel that " a 

 little may go a long way." 



It should be noted that the degree of visible complexity, even 

 in the microscopic nucleus of a germ-cell, is often very consider- 

 able. Thus Eisen observed in the nucleus of a species of sala- 

 mander twelve chromosomes, each of six parts, and in each part 

 six granules — altogether 432 visible units. 



