24 NATURE SKETCHES L\ TEMPERATE AMERICA 



it, and not in place of it. The scheme of tlie evohition of 

 species, through variation and heredity on the one hand, and 

 the selective influence of the environment on the other, has not 

 greatly changed since the date of the 'Origin of Species.' The 

 method, degree, and to some extent the causes of variation, 

 have been critically and successfully studied. The meaning 

 and the machinery of heredity have been the subject of most 

 fruitful investigation and experiment. Natural selection has 

 been subjected to the most searching analyses, and the fact 

 is that it remains the only general cause of the universal phe- 

 nomena of adaptation of life to environment. 



"Isolation has been separated from selection as a factor 

 theoretically distinct, but practically coexistent. The supposed 

 Lamarckian factors have disappeared, to reappear again in 

 unknown and perhaps unknowable forms. Theories of ele- 

 mental species, unit characters, and the Hke, have arisen to 

 meet the facts and guesses involved in the investigation of 

 mutation and the rediscovery of ]\Iendelism, taking their 

 place alongside of Darwin's bold hypothesis of pangenesis, to 

 pass away when the hypotheses are no longer needed. With 

 all this, on the whole, the scheme of organic evolution, as 

 presented in the 'Origin of Species,' still holds as an outline. 

 The work of fifty years has intensified the main features of the 

 sketch, and has constantly added to the work of the master, 

 without obliteration of any es.sential details." 



With all the advance made in biology up to the present 

 time, it may be stated that Darwin's work stands to-day as 

 an ideal for biological research the world over, and natural 

 selection, as Romanes says, "is the greatest idea that has ever 

 entered the mind of man." ^ 



' Those who are interested in organic evolution will find the following books 



of reference of particular value : 



Danrin, "Origin of Species," and "Descent of Man " 



Wallace, "Darwinism," and "Natural Selection and Tropical Nature." 



Romanes, "Darwin and .\fter Darwin," ami ".\n Examination of Weis- 



mannism." 



Ix)cl:, "Recent Progress in the Study of Variation, Heredity and Evolution." 



Huxley, "Evolution in Biology," Encyclopaedia Britannica. 



DeVries, "Species and Varieties," and "The Mutation Th«)ry" (English 



translation). 



Kellogg, "Evolution and .\nimal Life," and "Darwinism To-day." 



