546 



THE CAUSES OF EVOLUTION 



The Origin of Species by Natural Selection. — The Darwin- 

 ian Theory of Natural 

 Selection is outlined in 

 Wallace's Chart (Fig. 302). 

 This was originally pub- 

 lished in one of the vol- 

 umes by Alfred Russel 

 Wallace (1823-1913), who 

 was the co-discoverer with 

 Darwin of the selective 

 principle. 



Rapid Increase. — If we 

 elaborate the theory as 

 outlined by Wallace, there 

 is in all forms of life an 

 ever-present tendency to 

 Rapid Increase in Numbers. 

 This is shown by theoretical 

 calculations of the potential 

 rate of increase and by the 

 increase that actually 

 occurs when species invade 

 new territory that is es- 

 pecially suited to their 

 needs. According to theo- 

 retical calculations, a plant 



Fig. 301.— Alfred Russel Wallace (1823- 

 1913), who also proposed Natural Selec- 

 tion as a cause of organic evolution and 

 who was a lifelong friend of Darwin and 

 supporter of his views. 



(From Pirsson and Schuchert, "Textbook of 

 Geology," John Wiley & Sons, copyright, 1924, 

 reprinted by permission.) 



Wallace's Chart of Natural Selection 



Fig. 302. — The evidence and argument for Natural Selection as outhned by 



Alfred Russel Wallace. 



