2cp MORPHOLOGY 
cases the parent organism is supposed to give rise to progeny that vary 
in every direction, the successful direction to be determined by natural 
selection. This has been called indeterminate variation. In tracing 
the evolution of great groups, however, it becomes clear that the impor- 
tant variations occur in certain definite directions, which have been 
maintained persistently throughout all possible changes of condition. 
For example, the history of such a group as gymnosperms shows a ten- 
dency to vary in certain definite directions that has persisted from the 
early Paleozoic to the present time. What is true of the tendencies 
that result in great groups, has also been found to be true in many cases 
of related species. In other words, there is much to indicate that while 
variation may be indeterminate, there are also certain definite and pre- 
determined lines that persist. This origin of new forms (whether by 
natural selection or mutation or neither), as the result of a persistent 
determinate variation, is called orthogenesis. It certainly removes one 
of the greatest difficulties in the way of natural selection, and that is 
the beginning and development of a structure that can be of advantage 
only when completed. It satisfies also the many known cases 'of 
excessive development in certain directions, a development that may 
be not only disadvantageous, but even destructive. 
Even if determinate variation is accepted as a fact, however, what 
determines the persistent variation? The answer to this question has 
resulted in many variations of the theory of orthogenesis. In the earlier 
development of the theory, it was perhaps natural to explain it by means 
of a mysterious principle inherent in organic life, " an inner directive 
force " that persistently makes for progress. Of course such an " ex- 
planation " could not satisfy modern biologists, who prefer to believe 
that determinate variation is occasioned by external factors; but it is still 
very uncertain how these external factors operate, and even what they are. 
It should be noted that natural selection, mutation, and orthogenesis 
are not mutually destructive. They all deal with variations, and may 
all be operative in producing new forms. Natural selection deals with 
fluctuating variations, which are small and in every direction ; mutation 
with large variations, which are large and in every direction; and 
orthogenesis with those small or large and relatively few variations which 
for some reason persist and increase from generation to generation and 
carry forward the group as a whole. 
Weismannism. The theories of Weismann have strongly favored 
Darwin's theory of natural selection by supporting it at its weakest points. 
