A DEFINITION OF EVOLUTION 



who had reached the peak of his career during the Romantic Period. 

 Scientific interest in evolution was, indeed, at a low ebb. 



While evolutionary studies were largely suspended during this time, 

 studies in the many branches of biology \\'hich contribute to an under- 

 standing of evolution were actively pursued. The result was that the 

 stumbling blocks which caused the reaction were gradually removed, thus 

 paving the way for the Period of the Modern Synthesis. The most impor- 

 tant developments occurred in genetics. It became apparent that the large 

 mutations with which DeVries worked were rather rare, while much 

 smaller mutations, comparable to the individual fluctuations of which 

 Darwin wrote, were quite frequent. Further, the large mutations were 

 usually less viable than their normal alleles. Taxonomists, meanwhile, had 

 shown that natural species do not differ from one another by single, strik- 

 ing traits, but rather thev differ quantitativelv in a large number of traits. 



Study of wild species in the laboratory showed that pure lines are rare 

 in nature, being found usually only in self-fertilizing plants; hence the 

 pure line concept could no longer have a serious bearing on evolution. 

 Instead, it appeared that wild species are not only quite variable, but that 

 they commonly take up latent variability (heterozvgous recessive genes) 

 "like a sponge" ( Tschetverikoff, see Chapter 15). Geneticists and taxono- 

 mists both undertook the study of variability in wild species by the statis- 

 tical methods devised by Galton and extended by his successors. 



Finally, a new systematics has developed, in which the lower groups 

 are being studied by the methods of population genetics, ecology, physi- 

 ology, indeed, by every possible approach in addition to the classical 

 purely morphological approach, and all of this with a view to determining 

 the dynamics of the origin of species. On the higher taxonomic levels, an 

 effort is being made to apply to the problems of phylogeny the knowledge 

 which has been gained at the lower levels. 



THE PERIOD OF THE MODERN SYNTHESIS 



Thus the bases of the agnostic reaction were gradually destroyed, and 

 the Period of the Modern Synthesis naturally followed. The evolutionary 

 studies of this period have been marked bv confidence that the processes 

 of evolution are open to study as well as is the fact of evolution. On the 

 lower taxonomic levels genetic, ecological, geographical, and morphologi- 

 cal studies have all been brought to bear upon the problems of the origin 

 of hereditary variation and the origin of species. On the higher levels, 

 paleontologists especially have been applying the new knowledge gained 

 on the lower levels to the problems of phylogeny. While certain types of 

 study have been mentioned above as especiallv important, this is mislead- 

 ing, for the study of evolution at present is truK' the "modern svnthesis " 

 of all biological disciplines. It would be very difficult, to find an\' important 

 phase of biology which does not make some important contributions to 

 the study of evolution in the current Period ol the Modern Synthesis. 



A few men may be mentioned as leaders in llic present movement in 

 the studv of evolution. Th. ]3ob/hanskv deser\ cs first mention, for the 



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