2 GENETICS IN 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 
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