20 HEREDITY, AND THE ORIGIN OF SPECIES. 
and the organ may respond to other forces exclusively. 
Similar analyses must be used in the delineation of all 
unit-characters, and it is obvious, without further discus- 
sion, that we will never be able to uncover, either theo- 
retically or actually, more than a few of these indivisible 
units. The forms and activities by which we recognize 
plants must by no means be taken to be simple in their con- 
stitution unless proved to be so, and the modification of a 
character in hybridization and otherwise must be taken as 
proof that it is not an elementary feature. 
In tracing the development of species it is found prac- 
ticable to designate them as retrogressive when a distinct 
unit-capacity, or unit-character is lost or, rather, becomes 
latent. Thus the loss of color must be of this kind, and 
also the loss of geotropic reaction, while the power of form- 
ing laciniate leaves when shown by mutants from a simple- 
leaved type would be estimated as a progressive mutation, 
as the group of characters concerned belong to a more 
highly organized type of organ. Ifa white-flowered spe- 
cies should give rise to a form with colored flowers it 
might well be taken as a degressive movement, or as a 
retracing of a step once lost, since all flowers were in all 
probability originally colored. Here again the actual test 
is hybridization, it being accepted that the retrogressive 
forms of organs are recessive or latent when crossed with 
the parental or nearly related types. 
As to latency we can only say that strains of plants do 
carry capacities of one kind and another for many genera- 
tions without these particular forms of unit-characters 
showing any activity. Thus a mutant which springs from 
a parental type, and shows a laciniate leaf instead of the 
ancestral leaf, must carry the latter form in a latent con- 
dition. The latent character may be awakened from time 
to time, or may be entirely and permanently inactive. The 
most easily analyzable examples of latency are seen in 
