548 



THE CAUSES OF EVOLUTION 



have produced cells, similar to the original organism, to the 

 number represented by 2 raised to the 3029th power, or a volume 

 of protoplasm not less than lO^^*^^ times the volume of the earth. 

 These 3029 generations were produced in a period of five years. 

 Such rates of increase are never realized, but they exist potentially 

 in all animals and plants. 



Evidence that some such increase can actually occur is seen 



where animals and plants 

 spread rapidly over new 

 territory. The English 

 sparrow was first imported 

 into the United States 

 about 1850, and a few 

 were introduced later. 

 Within twenty-five years 

 it had spread and become 

 a pest. It is now our 

 most abundant and widely 

 distributed bird. The 

 German carp was intro- 

 duced from China to 

 Europe, and thence to the 

 United States, by impor- 

 tation of small numbers. 

 In the Mississippi Valley 

 alone it has now spread 

 into almost every stream, 

 to the limits of colder 

 water and other unsuit- 

 able conditions. The rab- 

 bits that have over-run 

 Australia were introduced, 



Fig. 304.— Louis Agassiz (1807-1873), pupil 



of Cuvier and distinguished Swiss zoologist, 



later professor at Harvard University. 



Unlike Huxloy and Wallace, Agassiz was unable 

 to appreciate the evidence for organic evolution, 

 although his own studies often present this evidence 

 in a striking manner (c/. Fig. 262, A to C). 



likewise the herds of wild 

 horses that were abundant in North and South America in the early 

 nineteenth century. There are so many instances of pests which 

 have been introduced and spread widely, that governments have 

 established quarantines against the importation of plants that might 

 be infested with insects, and there is a law in the United States 

 against the commercial importation of animals without inspection 

 by the Department of Agriculture. Other examples of actual in- 



