290 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. 



