2 'GENETICS I'N 'RELATION TO AGRICULTURE 



made it possible to group them into orders, families, genera, and species 

 according to the degree of resemblance which exists among groups of 

 individuals. But this is merely a view en masse of the differences be- 

 tween organisms, for it is universally true that no two individuals are 

 exactly alike. There are, therefore, for all practical purposes, two orders 

 of difference between individuals; first, racial differences, those which 

 separate groups of individuals, and second, individual differences, those 

 which distinguish the individuals of a group from one another. Strictly, 

 of course, there are all possible gradations from the one degree of dif- 

 ference to the other, but conveniently it may be said that the former, 

 the racial differences, are those which characterize different lines of 

 descent, whereas the latter, the individual differences, distinguish indi- 

 viduals within a given line of descent. The problem as to the origin of 

 racial differences is a problem of evolution; the problem of the origin of 

 individual differences is a problem of genetics, and we accordingly shall 

 construct our definition of variation to apply to differences exhibited 

 by individuals related by descent. 



Now all multicellular organisms which reproduce by sex exhibit the 

 common characteristic of two distinct cycles of cellular development; 

 gametogenesis, or development of the germ cells, and somatogenesis or 

 development of the body. The resemblances which make it possible 

 to group individuals into orders, families, genera, and species are the 

 result of the fundamental relation which exists between these two 

 cycles, for it is a commonplace fact that the germ cells of any species 

 can reproduce individuals of the same and no other species. This rela- 

 tion of germinal constitution to the development of the soma is specific 

 for all classes and grades of characters, but the order of specificity may 

 be either racial or individual, just as the order of difference between 

 individuals is racial or individual. 



The term variation carries with it the idea of deviation from type, 

 and obviously the above statements, brief as they are, of the cycles in 

 individual development leave room for several possibilities of deviation 

 from type. Thus, if we look at the matter from one point of view, the 

 guiding hand in determining the characters of the individual is the 

 specificity of the germinal substance. But every individual develops 

 under a certain set of conditions, the environment, which is independent 

 of the germinal substance; and these conditions have a certain, usually 

 merely modifying, influence in the development of the individual. 

 There is, therefore, a possibility for differences to arise in individuals 

 independently of differences in the germinal substance, differences which 

 are specifically attributable to diversities in the environment, and 

 which may have no effect on the germinal substance itself, just as the 

 degree of heat, for example, may cause a variation in the end products 



