THE HISTORY OF EVOLUTION 



17 



action of environment as a formative influence. The opinions of 

 later writers have carried on varying degrees of controversy to a 

 final recognition of the unproved state of both theories. In addi- 

 tion new theories have shown us that there are probably many 

 different processes of evolutionary change, as might be expected 

 in view of the great comphwity of living organisms. Natural selec- 

 tion and the inheritance of acquired characters must take? their 

 places with such 

 theories as muta- 

 tions and kineto- 

 genesis among the 

 numerous proba- 

 ble processes. 



Through the de- 

 velopment of the 

 science of genetics 

 accurate data 

 have been accu- 

 mulated on the 

 mechanism of 

 transmission of 

 characters which 

 must be involved 

 in the origin of 

 species as well as 

 in the origin of 

 individuals. The 

 facts available are 

 not yet wholly 

 correlated with 

 other fields of 

 l)iology but they 



are sufficient to furnish a sound foundation for future progress. 

 Best of all they encourage the broad thinking which alone can 

 arrive at great truths. Neither the philosophical nor the purely 

 materialistic aspects of evolution seem complete in themselves. 



Late years show an increased emphasis on the philosophical 

 aspects of the problem, and purposive evolution is now popular as 

 a modification of the older and more mechanistic th(^ories. The 

 chief problem of modern evolution, however we approach it, is 



Fig. 3. — Johann Gregor Mendel. (From Locy's Bi- 

 ology and It.s Makers, witli the permission of Henry 

 Holt and Company.) 



