The Origin of Species 207 



very long periods of time were allowed for its 

 complete action. It is evident that time must 

 have been an all-important factor if we are to 

 suppose, that by the interaction of the inherent 

 properties possessed by the elements of living 

 matter its structural arrangement became gradu- 

 ally modified in such a way, that existing classes 

 of animals and plants have been evolved out 

 of it. 



Charles Darwin's great work on the Origin 

 of Species was published in 1859, and was fol- 

 lowed by the first part of Herbert Spencer's 

 Principles of Biology and Huxley's Man's 

 Place in Nature, both of which appeared in the 

 year 1863; the evidence and opinions set forth 

 in the writings of these three Englishmen gave 

 a great impetus to scientists engaged in experi- 

 ment on the influence of mechanical, and other 

 forms of energy in modifying the character of 

 plants and animals. 



In the sixth edition of the Origin of Species, 

 published in 1872, Darwin had arrived at the 

 opinion, that the physical condition of organisms 

 lead to new sub-varieties without the aid of 

 selection. In a letter to a friend, dated 1876, 



